[Media Router] Fix URLRequestContext leak and race condition.

Remove usage of SystemURLRequestContextGetter which has led to several
issues including shutdown leak and test setup problems. This means 2
things:
- No more NetLog for DIAL and Cast sockets (for now. When we switch to
NetworkService we can reconsider adding back support depending on
our needs)
- DIAL device description / app info fetch now replaced with URLLoader
APIs that's part of network service.

The race condition issue caused by calling CMSSImpl::GetWeakPtr on two
different sequence is fixed by not using a WeakPtr by assuming
CMSSImpl outlives DialMSSImpl, which is true due to their ordering in
DualMediaSinkService. Also removed the PostTask trampoline by assuming
they both run on the same sequence.

Bug: 810676,805728,698940,779892,811226
Change-Id: Ib3ebb4f7f9d3690c952d2ddcbd579bb5d876906e
Reviewed-on: https://chromium-review.googlesource.com/912561
Commit-Queue: Derek Cheng <[email protected]>
Reviewed-by: Matt Menke <[email protected]>
Reviewed-by: Bin Zhao <[email protected]>
Cr-Commit-Position: refs/heads/master@{#538195}
34 files changed