[DevTools] Fix enabling/disabling emulation.

Changed PageResizer so it passes available size to
OverridesSupport itself, and has more control of when to do this.

BUG=327641

Review URL: https://codereview.chromium.org/349133003

git-svn-id: svn://svn.chromium.org/blink/trunk@176755 bbb929c8-8fbe-4397-9dbb-9b2b20218538