Docs: Add How to Write Cross-Platform UI doc

Change-Id: Ic1d89de8dd642e932b7917bea06dc1656aa8f539
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516075
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641666}
1 file changed