Standardized getStats() in chrome://webrtc-internals.

This CL replaces the stats displayed in in chrome://webrtc-internals
with the standardized metrics (https://w3c.github.io/webrtc-stats/).
This includes...
- Stats Tables
- Stats Graphs
- Create Dump > Download the PeerConnection updates and stats data

In addition to the raw stats, there are some calculated metrics added
to the results as well. This is because new metrics are typically
accumulative (total counters) but rates are usually much more
relatable. These are calculated every second.

The view of the old stats is still supported through a UI option where
you switch to "Read Stats From:" the legacy API. This affects stats
tables, graphs and dumps. The old mode should be unaffected by this CL.

Bug: 803014
Change-Id: Iaaf4fe0444be30a6300e4a8c94e0db486d54613f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565882
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653131}
4 files changed