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Adds MediaControlUi, which can be used to enable a simple touch UI for
media apps without a touch-supported UI. Some other changes are included
in this CL to simplify internal code as a result of this change:

* Remove CastWindowManager from CastContentWindow public API.
* Rename CastWindowManager::SetWindowId() to SetZOrder().
* Move CastContentWindow from "shell" namespace to "chromecast".

Merge-With: eureka-internal/316519

Bug: internal b/137663169
Test: CQ

Change-Id: I2d78e73162bb30a652f591b5def651468e44eaf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817146Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708712}