[Media Router] Do not remove sinks not found by AnyMediaSinksObserver

Until all MRPs support empty-source sink queries, QueryResultManager::
AnyMediaSinksObserver may return an incomplete list of sinks. This CL
makes QueryResultManager keep track of sinks with sources and sources
returned by AnyMediaSinksObserver separate, and take their set union
when notifying observers.

Bug: 929225
Change-Id: I33f8caf789ce8033c5481b9fa86691e9d7f3f537
Reviewed-on: https://chromium-review.googlesource.com/c/1459338
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632299}
6 files changed