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MediaSinkServiceBase changes:

- Move the Observer interface currently in CMSSImpl to
MediaSinkServiceBase and generalizes it so it can be used for other
MediaSinkServiceBase implementations.
- Standardizes add/update/remove/get sink operations in
MediaSinkServiceBase.
Both of these make the code using MediaSinkServiceBase more testable as
additional code in MRP depend on them.

Also cleaned up logic related to the discovery timer by combining
Start/RestartTimer().

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DialMediaSinkSevice changes:
- DMSSImpl now pools sinks discovered in the latest round and is used
to merge with the |MSSBase::sinks_| when the discovery timer fires.
Before this patch, the current set is removed on the start of a discovery
round (because unlike Cast, there is no active connection to a device to
tell that it is still alive or gone), which makes the sink list unstable.
With this patch, the current set is only modified at the end of a discovery
round.
- TODOs in future patches:
-- move app discovery service out of DMSSImpl, similar to Cast.
-- get rid of DMSS and move UI thread dependencies (if any) into part
   of DMSSImpl. Then rename DMSSImpl -> DMSS.

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CastMediaSinkService changes;
- No longer keep track of sinks by ip endpoint. Instead uses
|MSSBase::sinks_| to maintain sink list.
- CastAppDiscoveryService no longer need to maintain its own map of
sinks, and instead uses |MSSBase::GetSinks()/GetSinkById()|.
- The callback used for dual disovery is converted to use
MediaSinkServiceBase::Observer.
- TODOs in future patches:
-- get rid of CMSS and move UI thread dependencies into CMSSImpl.
Rename CMSSImpl -> CMSS.
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Notes:
- The MediaRouteDesktop ProvideSinks callback is not converted to use
Observers. This is because the callback is expected to be temporary (until
we can turn off the extension MRPs), so we treat it as a special case.

Bug: 816628,698940
Change-Id: I11fd13f7ebf743d78c9499ad24f98cf72f571ee5
Reviewed-on: https://chromium-review.googlesource.com/1033727
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555854}