Add a flag window.internals.testRunner.disableNotifyDone which ignores notifyDone() callbacks while it is true.

I would like to use this flag in GC fuzzers, where it runs existing LayoutTests inside an iframe. The notifyDone() callbacks from the target LayoutTests are ignored using this flag.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/18349008

git-svn-id: svn://svn.chromium.org/blink/trunk@154095 bbb929c8-8fbe-4397-9dbb-9b2b20218538