Add Blink APIs for frame tree mirroring.

public/web/WebFrame.h

Index: public/web/WebFrame.h
diff --git a/public/web/WebFrame.h b/public/web/WebFrame.h
index a05f137c92a21cd743c39debd98e4c93f8ef66a7..cf25a04cc2dc4605cd720cca2c1a804e0d694ef1 100644
--- a/public/web/WebFrame.h
+++ b/public/web/WebFrame.h
@@ -201,6 +201,9 @@ public:
     // This is executed between layout tests runs
     void clearOpener() { setOpener(0); }
 
+    // Called by the WebView to initialize the main frame for the page.
+    virtual void initializeAsMainFrame(WebView*) = 0;

[abarth-chromium, 2014/06/13 18:06:46]

Why is this API public?  If it's called by the WebView, it can be private,
right?

[dcheng, 2014/06/13 23:56:12]

I'd like to avoid friending WebViewImpl if possible--it seems a bit out of place
here.

How would you feel about de-virtualizing this, and updating
WebViewImpl::setMainFrame to dispatch like this:
    if (f->isLocal())
        toWebLocalFrameImpl(f)->initializeAsMainFrame(page());
    else
        toWebRemoteFrameImpl(f)->initializeAsMainFrame(page());

+
     // Adds the given frame as a child of this frame.
     virtual void appendChild(WebFrame*);