Refactoring: Add toWebFrameImpl() interface.

We should avoid static_cast at various places by using proper conversion function. This would avoid the error that can be introduced with each manual addition of static_cast addition. Unlike other toClassName functions (like in rendering), we do not require an assertion to confirm the type of passed WebFrame object, since WebFrameImpl is the only child class of WebFrame.

BUG=NONE
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23671002

git-svn-id: svn://svn.chromium.org/blink/trunk@156825 bbb929c8-8fbe-4397-9dbb-9b2b20218538