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This change enables future UI changes based on the
'window-controls-overlay' display override by adding a new method,
`IsWindowControlsOverlayEnabled()`,to the web app controller that
indicates whether or not the WCO should be shown.

Explainer: https://github.com/WICG/window-controls-overlay/blob/master/explainer.md
Design Doc: https://docs.google.com/document/d/1k0YL_-VMLIfjYCgJ2v6cMvuUv2qMKg4BgLI2tJ4qtyo/edit?usp=sharing
I2P: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/cper6nNLFRQ/hU91kfCWBQAJ

Bug: 937121
Change-Id: Ibe5615393e50edc09e2464abe9eaa09e52559be6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2486780Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Lorne Mitchell <lomitch@microsoft.com>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Amanda Baker <ambake@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#821037}