Add RoundedCornerDecorator and apply it to Picture-in-Picture windows.

    I tested: 1. open pip window, 2. move around, 3. resize, 4. close,
    5. re-open, 6. close tab destroying the window
    off.

Bug: 841886
Test: Chrome PIP works with rounded corners flag both on and off.
Test: Android PIP window works with rounded corners flag both on and
Change-Id: Iec59abde476d56c58e197b9220ac6ba418801395
Reviewed-on: https://chromium-review.googlesource.com/c/1347969
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611957}
12 files changed