GitLab

This way the WindowPort can reach back into the Window during destruction.

BUG=847992
TEST=covered by tests

Change-Id: I70df1c9512086f9e834ab76fa430c94f5723f34d
Reviewed-on: https://chromium-review.googlesource.com/1175133Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583163}