Framework for converting embedder uses of WebFrame to WebLocalFrame.

This patch adds a typedef for WebFrame so we can start converting
references to WebFrame in embedder implementations. It also updates most
references of WebFrame in the embedding API to WebLocalFrame instead.
Most locations should only ever have to handle WebLocalFrames, so it's
just a fairly straightforward rename. However, there are some corner
cases that need to be adjusted more carefully, since things like the
opener, parent frame, and child frames can be pointers to an eventual
WebRemoteFrame class.

BUG=346764

Review URL: https://codereview.chromium.org/191003006

git-svn-id: svn://svn.chromium.org/blink/trunk@169638 bbb929c8-8fbe-4397-9dbb-9b2b20218538