Split fullscreen controller from WebViewImpl

WebViewImpl contains too much code. This patch moves fullscreen-related code
to a seperate helper class FullscreenController.

There was a core::FullscreenController that implements the WHATCG Fullscreen API. That class is renamed to FullscreenElementStack.

No tests needed. Does not change behavior.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21316002

git-svn-id: svn://svn.chromium.org/blink/trunk@155358 bbb929c8-8fbe-4397-9dbb-9b2b20218538