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AppRegistrar should be used by clients to query information about apps.
For example, if an app is installed.

AppRegistrar has two implementations BookmarkAppRegistrar and
WebAppRegistrar. BookmarkAppRegistrar is built on top of extensions.

PendingBookmarkApp uses BookmarkAppRegistrar to avoid querying the
ExtensionRegistry and ExtensionPrefs directly.

Bug: 916381

Change-Id: I0706bef37f2ecbf4b86d63eafe8de1dd168f440a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1498736
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637598}