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This CL deletes the extensions::BookmarkOrHostedAppInstalled() method,
and adds an abstract IsInstalled(const GURL& start_url) method to the
AppRegistrar to replace it. The new method's concrete implementation
lives in BookmarkAppRegistrar (migrated from its old home).

The existing call sites for the method are migrated as well.

BUG=891172

Change-Id: Ide93d8f6e799ad325ae8b8902b43787c89636601
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585281
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654777}