Use idle task to perform incremental marking steps.

src/v8.h

Index: src/v8.h
diff --git a/src/v8.h b/src/v8.h
index 71dd42f49df8dae580a36492e306a07432d728d5..fefe381fa3027679defb946c7439ee13d0576f29 100644
--- a/src/v8.h
+++ b/src/v8.h
@@ -72,6 +72,9 @@ class V8 : public AllStatic {
   static void InitializePlatform(v8::Platform* platform);
   static void ShutdownPlatform();
   static v8::Platform* GetCurrentPlatform();
+  // Replaces the current platform with the given platform.
+  // Should be used only for testing.
+  static void SetPlatformForTesting(v8::Platform* platform);
 
   static void SetNativesBlob(StartupData* natives_blob);
   static void SetSnapshotBlob(StartupData* snapshot_blob);