Adds metrics for RTCQuicTransport origin trial.

The metrics inludes usage counters for the RTCQuicTransport,
RTCIceTransport and RTCQuicStream APIs. This also includes histogram
metrics for the readInto() and write() RTCQuicStream methods, recording
the amount of bytes as well as an enum for how bytes are read/written.

Bug: 874296
Change-Id: I53036a9e0c477b98ab52e32484b1fb472140c305
Reviewed-on: https://chromium-review.googlesource.com/c/1435074
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Steve Anton <steveanton@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Seth Hampson <shampson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625970}
9 files changed