Refactoring: Add toWebFrameImpl() interface.

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index 9c7f9baa726108aa84e977ab7fd54c5e872ebff1..c802861b8e83491283dba3b29d541f153b5c07bd 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -2586,7 +2586,7 @@ WebFrame* WebViewImpl::findFrameByName(
 {
     if (!relativeToFrame)
         relativeToFrame = mainFrame();
-    Frame* frame = static_cast<WebFrameImpl*>(relativeToFrame)->frame();
+    Frame* frame = toWebFrameImpl(relativeToFrame)->frame();
     frame = frame->tree()->find(name);
     return WebFrameImpl::fromFrame(frame);
 }
@@ -2605,8 +2605,7 @@ void WebViewImpl::setFocusedFrame(WebFrame* frame)
             frame->selection()->setFocused(false);
         return;
     }
-    WebFrameImpl* frameImpl = static_cast<WebFrameImpl*>(frame);
-    Frame* webcoreFrame = frameImpl->frame();
+    Frame* webcoreFrame = toWebFrameImpl(frame)->frame();
     webcoreFrame->page()->focusController().setFocusedFrame(webcoreFrame);
 }