Add Picture-in-picture specific window resizer.

Currently, this does essentially nothing apart from providing basic
resizing and moving functionality. In a follow-up CL, we can add
snap-to-edge, swipe-to-dismiss, and interaction with system ui regions
like the virtual keyboard.

Bug: 841886
Bug: b/115291749
Test: Able to move and resize an Android PIP window.
Change-Id: Iaecc43154fd38dc5f9d3b83549e97dc44a89d749
Reviewed-on: https://chromium-review.googlesource.com/c/1219428
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599172}
7 files changed