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This change adds getters for individual ArcServices from
ArcServiceManager.  This prevents ArcServiceManager::Observer from
becoming a place to put random stuff to observe that would be better
suited for individual services to expose.

BUG=672840
TEST=git cl try

Review-Url: https://codereview.chromium.org/2622843002
Cr-Commit-Position: refs/heads/master@{#442934}