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- In the header file, change TileServiceScheduler to a pure interface,
and create TileServiceSchedulerImpl class to inherit from LogSource and
TileServiceScheduler both.

- Implemented LogSource in Scheduler - as TileServiceImpl will actually
owns the Logger.

- Create Delegate in scheduler and GetTileGroup through TileManager,
in order to expose to LogSource.

- Added two more scheduler event APIs OnFetchStarted and OnGroupDataSaved
in order to update the internal status.

- (TODO) convert TileGroup to base::Value with pretty format.

- Follow up CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2264934

Bug: 1066556
Change-Id: Ifa8099499f17e0cddf745dffc7384b3e3595fa2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277073
Auto-Submit: Hesen Zhang <hesen@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Hesen Zhang <hesen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785892}