Prepare a transparent window and pass the fullscreen handwriting feature to IME

Make a transparent window, and use a zero overlap in InitInsets and UpdateInsetsForWindow
to avoid the window beneath not rendering anything.

Bug: 826617
Change-Id: I9b5e8140a6b9a9c481caf92ef3d244ffc14b64d8
Reviewed-on: https://chromium-review.googlesource.com/1013444
Commit-Queue: Jianzhou Feng <jzfeng@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551592}
4 files changed