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When more components use AppService, the public interfaces without
comments could be very confused to use. So add the comments to
AppServiceProxy's public interfaces.

BUG=1016159

Change-Id: Ibf7470a356b15cafccc5def2fd478e79914f5fb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2009594Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733486}