Replace the RenderViewHostObserver in UIThreadExtensionFunction with a WebContentsObserver. This is part of the effort to remove RenderViewHostObserver.

BUG=306569
R=yoz@chromium.org

Review URL: https://codereview.chromium.org/32633004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230037 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230037