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Adds methods to SnapshotController and SnapshotController::Client for
the renovation framework. Also adds basic implementations to all
SnapshotController::Client implementors. This doesn't change the
behavior of the SnapshotController yet.

Design doc:
https://docs.google.com/document/d/1WuwRJaxBKAwVpeAHG0xhq5uRxMTB-j4LFleT4QEqANU

Bug: 736933
Change-Id: Iee15c54fa4524b1c09a87f6e6c43a8df8ea630ee
Reviewed-on: https://chromium-review.googlesource.com/580692Reviewed-by: Peter Williamson <petewil@chromium.org>
Reviewed-by: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Yafei Duan <romax@chromium.org>
Commit-Queue: Collin Baker <collinbaker@google.com>
Cr-Commit-Position: refs/heads/master@{#488696}