Oilpan: Expose Heap::collectGarbage to the web layer

public/web/WebHeap.h

Index: public/web/WebHeap.h
diff --git a/public/web/WebHeap.h b/public/web/WebHeap.h
index b8d01b3af2b514f86d2fea7a4044bb85a236d917..fa4fdaed909153fab54ded45488f903e86170cab 100644
--- a/public/web/WebHeap.h
+++ b/public/web/WebHeap.h
@@ -47,6 +47,11 @@ 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