Navigation transitions: Added notifyTransitionsShown and setupTransitionsView to WebLocalFrame

The former is called on the outgoing page, and will hide any transition elements and enable any exit transition animations.

The latter is called on the transition layer, and will replace the document (about:blank) with the serialized markup, enforce sandboxing, and turn off script execution.

R=eseidel,esprehn
BUG=370696

Review URL: https://codereview.chromium.org/316053007

git-svn-id: svn://svn.chromium.org/blink/trunk@179431 bbb929c8-8fbe-4397-9dbb-9b2b20218538