Plumbing in preparation for standardized getStats() in webrtc-internals.

This CL implements the calling chain from webrtc_internals.js all the
way to getStats() and back, including process hops and layering.
- This adds the calling chain for standard getStats() API. The original
calling chain for legacy getStats() remains intact, but is renamed.

rtc_peer_connection_handler.cc implements invoking the standard API and
formats the result in the way that webrtc_internals.js expects stats to
be formatted.

These are roughly the layers:
- webrtc_internals.js "get[Legacy/Standard]Stats"
- WebRTCInternalsMessageHandler::OnGet[Legacy/Standard]Stats
- PeerConnectionTracker::OnGet[Legacy/Standard]Stats
- webrtc::PeerConnectionInterface::GetStats
- Internal[Legacy/Standard]StatsObserver::[OnComplete/OnStatsDelivered]
- PeerConnectionTrackerHost::OnAdd[Legacy/Standard]Stats
- WebRTCInternals::OnAdd[Legacy/Standard]Stats
- WebRTCInternalsMessageHandler::OnUpdate
- webrtc_internals.js "add[Legacy/Standard]Stats"

This CL is split up from
https://chromium-review.googlesource.com/c/chromium/src/+/1536186
to ease reviewing. The new calling chain is NOT exercised in this CL,
this will be handled in a follow-up CL.

Bug: 803014
Change-Id: Ied4b0b8f136f92aaad4633edb38ccb288e41ef3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565327
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653118}
14 files changed