[CCA WebUI] Implement CameraAppWindowManager

For multiple window management, we implement CameraAppWindowManager
which will has a instance for each browser context. When a window is
created, it changed the camera resource ownership and notify inactive
windows to suspend current camera stream.

Bug: 980846
Test: Tested with the latter CLs to make sure CCA works normally for
multi-window scenarios.

Change-Id: Ice92e321a8973c54c5a20ef4ac0759560ad49fed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2345898
Commit-Queue: Wei Lee <wtlee@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Inker Kuo <inker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822598}