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This is the first CL to move memory_coordinator outside components/.
Also, per discussion on [1], this CL removes MemoryCoordinatorClient
from HistoryBackend. We can re-add it if needed.

[1] https://codereview.chromium.org/2297243004/#msg14

BUG=644022

Review-Url: https://codereview.chromium.org/2311723002
Cr-Commit-Position: refs/heads/master@{#418766}