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CastContentWindowImpl is the implementation of CastContentWindow
now. Add a CastContentWindowAndroid specific for Android platform.

BUG=NONE
TEST=cast_shell_apk

Change-Id: I6092f733b54638256bd3028ac3d889f6b52a02fe
Review-Url: https://codereview.chromium.org/2570623003
Cr-Commit-Position: refs/heads/master@{#443358}