Add mono_gc_start_incremental_collection API (#1297)

* update bdwgc

* Expose mono_gc_start_incremental_collection API

* switch bdwgc repo back to unity-master branch

Author: jechter

mono/metadata/boehm-gc.c

@@ -336,6 +336,11 @@ mono_gc_collect_a_little()
 	return GC_collect_a_little();
 }
 
+void mono_gc_start_incremental_collection()
+{
+	GC_start_incremental_collection();
+}
+
 /**
  * mono_gc_max_generation:
  *

mono/metadata/mono-gc.h

@@ -125,6 +125,7 @@ MONO_API MonoBoolean mono_gc_is_incremental (void);
 MONO_API void 	mono_gc_set_incremental(MonoBoolean value);
 MONO_API void     mono_gc_finalize_notify    (void);
 MONO_API int    mono_gc_invoke_finalizers (void);
+MONO_API void mono_gc_start_incremental_collection();
 /* heap walking is only valid in the pre-stop-world event callback */
 MONO_API int    mono_gc_walk_heap        (int flags, MonoGCReferences callback, void *data);
 

mono/metadata/null-gc.c

@@ -61,6 +61,11 @@ mono_gc_collect (int generation)
 {
 }
 
+void
+mono_gc_start_incremental_collection()
+{
+}
+
 int
 mono_gc_max_generation (void)
 {

mono/metadata/sgen-mono.c

@@ -2620,6 +2620,12 @@ mono_gc_collect (int generation)
 	MONO_EXIT_GC_UNSAFE;
 }
 
+void 
+mono_gc_start_incremental_collection()
+{
+
+}
+
 int
 mono_gc_collection_count (int generation)
 {