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This CL creates a separate class that is in charge of recording UMAs on behalf
of TabManager for tab and system-related properties and events during session
restore and background tab loading. The RecordSwitchTo metric method has been
moved out of TabManager and into TabManagerStatsCollector.

In this implementation, the TabManagerStatsCollector is scoped to the lifetime
and owned by the TabManager, and the TabManagerStatsCollector holds a pointer
to the TabManager.

Bug: 740625
Change-Id: Ic3c77d5d99563ceefedab3d417721a7624d02ffe
Reviewed-on: https://chromium-review.googlesource.com/565353Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Zhen Wang <zhenw@chromium.org>
Commit-Queue: Scott Haseley <shaseley@google.com>
Cr-Commit-Position: refs/heads/master@{#487311}