[Fizz] Gate legacyContext field on disableLegacyContext (#30173)

We're running out of fields and this one we can avoid at runtime in any
modern builds.

DiffTrain build for [6d2a97a](6d2a97a)

Author: sebmarkbage

compiled/facebook-www/REVISION

@@ -1 +1 @@
-315109b02b0c9460b7466ca88f3f4d6ed1215a21
+6d2a97a7113dfac2ad45067001b7e49a98718324

compiled/facebook-www/REVISION_TRANSFORMS

@@ -1 +1 @@
-315109b02b0c9460b7466ca88f3f4d6ed1215a21
+6d2a97a7113dfac2ad45067001b7e49a98718324

compiled/facebook-www/React-dev.classic.js

@@ -1991,7 +1991,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.0.0-www-classic-315109b02b-20240701";
+    exports.version = "19.0.0-www-classic-6d2a97a711-20240701";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-dev.modern.js

@@ -1971,7 +1971,7 @@ __DEV__ &&
     exports.useTransition = function () {
       return resolveDispatcher().useTransition();
     };
-    exports.version = "19.0.0-www-modern-315109b02b-20240701";
+    exports.version = "19.0.0-www-modern-6d2a97a711-20240701";
     "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
       "function" ===
         typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-prod.classic.js

@@ -669,4 +669,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-classic-315109b02b-20240701";
+exports.version = "19.0.0-www-classic-6d2a97a711-20240701";

compiled/facebook-www/React-prod.modern.js

@@ -669,4 +669,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-modern-315109b02b-20240701";
+exports.version = "19.0.0-www-modern-6d2a97a711-20240701";

compiled/facebook-www/React-profiling.classic.js

@@ -673,7 +673,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-classic-315109b02b-20240701";
+exports.version = "19.0.0-www-classic-6d2a97a711-20240701";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-profiling.modern.js

@@ -673,7 +673,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-modern-315109b02b-20240701";
+exports.version = "19.0.0-www-modern-6d2a97a711-20240701";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/ReactART-dev.classic.js

@@ -16934,14 +16934,14 @@ __DEV__ &&
         scheduleRoot: scheduleRoot,
         setRefreshHandler: setRefreshHandler,
         getCurrentFiber: getCurrentFiberForDevTools,
-        reconcilerVersion: "19.0.0-www-classic-315109b02b-20240701"
+        reconcilerVersion: "19.0.0-www-classic-6d2a97a711-20240701"
       });
     })({
       findFiberByHostInstance: function () {
         return null;
       },
       bundleType: 1,
-      version: "19.0.0-www-classic-315109b02b-20240701",
+      version: "19.0.0-www-classic-6d2a97a711-20240701",
       rendererPackageName: "react-art"
     });
     var ClippingRectangle = TYPES.CLIPPING_RECTANGLE,

compiled/facebook-www/ReactART-dev.modern.js

@@ -16364,14 +16364,14 @@ __DEV__ &&
         scheduleRoot: scheduleRoot,
         setRefreshHandler: setRefreshHandler,
         getCurrentFiber: getCurrentFiberForDevTools,
-        reconcilerVersion: "19.0.0-www-modern-315109b02b-20240701"
+        reconcilerVersion: "19.0.0-www-modern-6d2a97a711-20240701"
       });
     })({
       findFiberByHostInstance: function () {
         return null;
       },
       bundleType: 1,
-      version: "19.0.0-www-modern-315109b02b-20240701",
+      version: "19.0.0-www-modern-6d2a97a711-20240701",
       rendererPackageName: "react-art"
     });
     var ClippingRectangle = TYPES.CLIPPING_RECTANGLE,