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This change adds support for the display_override property to the
webapp manifest. This change proposes that we reuse the existing
DisplayMode enum. It adds the field to manifest.mojom, as well
as parsing of the field.

This change proposes that we ignore unknown entries, and that
the overrides are stored in order.

Explainer: https://github.com/WICG/display-override/blob/master/explainer.m
Design document: https://docs.google.com/document/d/1hEmbGVHMN38q1YTaaGccQ-Y5CHr7xIURYPRWXTuvZLo/edit?usp=sharing
I2P: https://groups.google.com/a/chromium.org/d/topic/blink-dev/WvIeZT8uSzw/discussion

Bug: 1092667
Change-Id: I772abbadf00f32cad2fbd62280ebcee7b4616702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274170
Commit-Queue: Mike Jackson <mjackson@microsoft.com>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788625}