gesture-nav: Use a WebContentsObserver to more reliably dismiss the navigation overlay.

WebContentsObserver provides the necessary notifications to know when to remove the
screenshot overlay during gesture-navigation. So use those instead of PaintObserver.

BUG=340751
R=sky@chromium.org

Review URL: https://codereview.chromium.org/153913004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249073 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249073