Cleanup: Add more conversion helpers for usage.

Source/web/WebFrameImpl.cpp

Index: Source/web/WebFrameImpl.cpp
diff --git a/Source/web/WebFrameImpl.cpp b/Source/web/WebFrameImpl.cpp
index 3e4ebcebfc24701d57a41c1e1280d42219d5cd05..f36023e334808e3c5454d91860318028c4fb5ba0 100644
--- a/Source/web/WebFrameImpl.cpp
+++ b/Source/web/WebFrameImpl.cpp
@@ -2255,7 +2255,7 @@ WebFrameImpl* WebFrameImpl::fromFrame(Frame* frame)
 {
     if (!frame)
         return 0;
-    return static_cast<FrameLoaderClientImpl*>(frame->loader()->client())->webFrame();
+    return toFrameLoaderClientImpl(frame->loader()->client())->webFrame();
 }
 
 WebFrameImpl* WebFrameImpl::fromFrameOwnerElement(Element* element)
@@ -2275,12 +2275,12 @@ WebViewImpl* WebFrameImpl::viewImpl() const
 
 WebDataSourceImpl* WebFrameImpl::dataSourceImpl() const
 {
-    return static_cast<WebDataSourceImpl*>(dataSource());
+    return toWebDataSourceImpl(dataSource());
 }
 
 WebDataSourceImpl* WebFrameImpl::provisionalDataSourceImpl() const
 {
-    return static_cast<WebDataSourceImpl*>(provisionalDataSource());
+    return toWebDataSourceImpl(provisionalDataSource());
 }
 
 void WebFrameImpl::setFindEndstateFocusAndSelection()