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Ash(ShelfController) should observe AppRegistryCache to get the app
notification badging information. Ash can't depend on chrome/*, so we
can't use AppServiceProxy to get AppRegistryCache, and we can't use
profile as well, because that is in chrome/browser/chromeos directory.

Create a new AppRegistryCacheWrapper to map account id to
AppRegistryCache, so that Ash or other external browser components can
access AppRegistryCache. This could help offload AppServiceProxy, extend
AppRegistryCache to be used out of chrome/browser, and benefit for the
future if we have an AppRegistryCache out of AppServiceProxy.

BUG=1068884

Change-Id: Ib5caa94d944a0d031df9dbee0e3d2c00587ab402
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212285
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772324}