Rename WebFrameImpl to WebLocalFrameImpl.

WebFrameImpl is going to be the concrete implementation of WebLocalFrame
going forward. A followup patch will introduce its counterpart for
remote frames, WebRemoteFrameImpl.

BUG=346764

Review URL: https://codereview.chromium.org/241303002

git-svn-id: svn://svn.chromium.org/blink/trunk@171986 bbb929c8-8fbe-4397-9dbb-9b2b20218538