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Currently fcm channel and subscription to topics use same enum in order to track their
state. This Cl introduces separate enum for the Subscription channel state tracking.

Bug: 801985, 878446

Change-Id: If5a4781fdd2e12b8d94245375a1480e237248321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505441Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638585}