Do V8 GC ASAP if system memory is pressured[reland]

content/renderer/render_thread_impl.h

Index: content/renderer/render_thread_impl.h
diff --git a/content/renderer/render_thread_impl.h b/content/renderer/render_thread_impl.h
index 806d32bfbd261725bf64de15bf2db7ae1358d6c4..039455b9f4c89daad84afdcc8a664a1937778e0d 100644
--- a/content/renderer/render_thread_impl.h
+++ b/content/renderer/render_thread_impl.h
@@ -515,6 +515,9 @@ class CONTENT_EXPORT RenderThreadImpl
 
   void ReleaseFreeMemory();
 
+  void OnSyncMemoryPressure(
+      base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level);
+
   // These objects live solely on the render thread.
   std::unique_ptr<AppCacheDispatcher> appcache_dispatcher_;
   std::unique_ptr<DomStorageDispatcher> dom_storage_dispatcher_;