GitLab

So far the display ID was recognizesed on creation of a toplevel,
but moving across displays isn't considered well.

This CL recognizes the timing of a window moving to another
display, and let WindowTreeHostMus updates its own display ID
properly.

BUG=866947
TEST=the new test case covers

Change-Id: I1e859f879a919674e90875a74434bb73e94edd73
Reviewed-on: https://chromium-review.googlesource.com/1234857
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593013}