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With this in place, we can call AppShimRegistry from
BookmarkAppShortcutManager instead of AppShortcutManager.

Note that some tests are built around extensions, and will
need to be substantially modified. Those are moved to
platform_apps, because the new bookmark-based versions will
continue to exist alongside the old extensions-based
versions.

R=loyso
TBR=avi (chrome/)

Bug: 1057195
Change-Id: I4784eedcb270fb679e12375d3a729107ada37f11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2109272
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751930}