chromeos: don't allocate LocalSurfaceId if size doesnt change

If WindowTreeHostMus::SetBounds() is called with no LocalSurfaceIdAllocation
(as happens when views calls SetBounds()) and the size hasn't changed, then
the current LocalSurfaceId should be used.

BUG=931878
TEST=covered by test

Change-Id: I135dca75ce04220796c41dd9d49042127d5dcfc5
Reviewed-on: https://chromium-review.googlesource.com/c/1474252
Auto-Submit: Scott Violet <sky@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632394}
2 files changed