Oilpan: Expose Heap::collectGarbage to the web layer

public/web/WebHeap.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 29 matching lines @@
     // While this object is active on the stack current thread is marked as
     // being at safepoint. It can't manipulate garbage collector managed objects
     // until it leaves safepoint, but it can block indefinitely.
     // When thread is not at safe-point it must not block indefinitely because
     // garbage collector might want to stop it.
     class SafePointScope {
     public:
         BLINK_EXPORT SafePointScope();
         BLINK_EXPORT ~SafePointScope();
     };
+
+    // These APIs are only for testing purposes and should not be used
+    // outside of tests.
+    BLINK_EXPORT static void collectGarbageForTesting();
+    BLINK_EXPORT static void collectAllGarbageForTesting();
 };
 
 } // namespace blink
 
 #endif