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This work is to support more generic rendering for mobile platforms.
The thinking behind using a stack of 'drawable' objects is we could
add future features like fps stats or more animations, and also add
debug screens to apps in development for gesture feedback and connection
health.

R=yuweih@chromium.org
BUG=671692

Review-Url: https://codereview.chromium.org/2591363002
Cr-Commit-Position: refs/heads/master@{#443246}