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This change fixes the icon existence checking for web apps in
chrome://apps to use downloaded icons instead of icon_info's from the
manifest.

R=loyso@chromium.org

Fixed: 1062987, 1078695
Change-Id: If27239cf1e4901369c0ab52182cdcf7f78c9982a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2183118Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#766532}