[Oilpan] Add a GC_PLUGIN_IGNORE with a TODO comment

Source/web/WebDevToolsFrontendImpl.h

Index: Source/web/WebDevToolsFrontendImpl.h
diff --git a/Source/web/WebDevToolsFrontendImpl.h b/Source/web/WebDevToolsFrontendImpl.h
index 83de9fd35ea2b6a70b08d69c887deeeaf646c3a4..ceceb67eb03a482f20ddc72b4cae10f0639c61f8 100644
--- a/Source/web/WebDevToolsFrontendImpl.h
+++ b/Source/web/WebDevToolsFrontendImpl.h
@@ -62,6 +62,8 @@ public:
     void setInjectedScriptForOrigin(const String& origin, const String& source) override;
 
 private:
+    // TODO(Oilpan): Make the lifetime of m_webFrame clear to remove this GC_PLUGIN_IGNORE.
+    GC_PLUGIN_IGNORE("http://crbug.com/509911")
     WebLocalFrameImpl* m_webFrame;
     WebDevToolsFrontendClient* m_client;
     RefPtrWillBePersistent<DevToolsHost> m_devtoolsHost;