Close mirroring window only when necessary

always recreating was causing extra XRANR event on X11 system.

Delete WindowTreeHost asynchronously when the change can be made via input event.

BUG=484580,486190
TEST=tested manually. no flicker.

Review URL: https://codereview.chromium.org/1133323002

Cr-Commit-Position: refs/heads/master@{#329761}
7 files changed