Rename V8WindowShell to WindowProxy

V8WindowShell is an implementation of "WindowProxy" in the spec
(http://www.w3.org/TR/2009/WD-html5-20090423/browsers.html#windowproxy).
We should rename it accordingly.

Review URL: https://codereview.chromium.org/451013003

git-svn-id: svn://svn.chromium.org/blink/trunk@180146 bbb929c8-8fbe-4397-9dbb-9b2b20218538