Implement ChildThread::shutdown and clear members that have refs into blink

      This enables classes deriving frmo ChildThread to cleanly shut down
WebKit

BUG=263034
R=jam@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19641008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@213239 0039d316-1c4b-4281-b951-d872f2087c98