In multi-user mode on chromeos, we check whether a new window was
created by a user gesture by looking at the state of the MessageLoop and
the EventDispatcher. This doesn't work in an asynchronous world.

This patch replaces this check by manually annotating windows which were
created by user gesture.

BUG=690160
TBR=stevenjb@chromium.org

Review-Url: https://codereview.chromium.org/2685333005
Cr-Commit-Position: refs/heads/master@{#452222}