Make the destructor of WebFrameClient virtual.

Most subclasses of WebFrameClient assume that the destructor of WebFrameClient
is virtual. Currently, CreateChildCounterFrameClient class in WebViewTest.cpp
is in trouble because of this.

BUG=N/A

Review URL: https://codereview.chromium.org/320583002

git-svn-id: svn://svn.chromium.org/blink/trunk@175685 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/public/web/WebFrameClient.h

@@ -506,7 +506,7 @@ public:
     virtual void initializeChildFrame(const WebRect& frameRect, float scaleFactor) { }
 
 protected:
-    ~WebFrameClient() { }
+    virtual ~WebFrameClient() { }
 };
 
 } // namespace blink