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This CL switches the interface between ArcBridgeBootstrap
and ArcBridgeServiceImpl from Delegate to Observer.
Also, ArcBridgeHost will be instantiated in ArcBridgeBootstrap
and kept in it.

BUG=633258
TEST=Trybots.

Review-Url: https://codereview.chromium.org/2379223004
Cr-Commit-Position: refs/heads/master@{#423051}