[Backport] Remove X11 and GTK dependencies from host_forwarder

Use libevent as the implementation of MessagePumpForUI.

BUG=346113

Review URL: https://codereview.chromium.org/176713005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257853 0039d316-1c4b-4281-b951-d872f2087c98