Allow PipWindowKey for SingleProcessMash

This window property is used to indicate a window is picture-in-
picture mode; without this property, incorrect window resizer
will be used.

Bug: 921156
Test: manually
Change-Id: I31e8f11378ee2ab862f4c3217f5ad94acd1dc9dc
Reviewed-on: https://chromium-review.googlesource.com/c/1409690
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622609}
3 files changed