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StopReason is passed to following observer methods:
- ArcBridgeBootstrap::Observer::OnStopped
- ArcBridgeService::Observer::OnBridgeStopped

Those callbacks can now tell the reason the bridge stopped.

I am planning to use this soon to show notifications on failures.
See the bug entry for more details.

BUG=chromium:625923
TEST=components_unittests --gtest_filter=ArcBridgeTest.*
TEST=Manually verified that ARC boots.

Review-Url: https://codereview.chromium.org/2133653002
Cr-Commit-Position: refs/heads/master@{#405241}