Mojo: Add simple windowing in mojo/core/ports.

Change-Id: I1cbd569582523db89f1537c137215f12be466708
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1680721
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#674513}
7 files changed