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When web apps were using the Extension system it got icon updating for
free. Now that BMO is enabled and there's no underlying Extension app
manifest updates must trigger the shortcut icon updating explicitly.

This change affects Linux, Windows and Mac. Chrome OS icon updating
requires app service integration to be followed up with in a later CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2222180/1

Bug: 1084405
Change-Id: Id22327069d2b9ed5f2381d2c159846417d79afab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216143
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773589}