Populate RTCStatsReport directly to V8 objects instead of dictionary

This saves from needing to copy the dictionary keys for each getStats
call. From local testing this removes around 48k character copied,
which is 96kB of UTF-16 data.


Binary-Size: Size increase due to IDL import.
Fuchsia-Binary-Size: Size increase due to IDL import.
Bug: 1414363
Change-Id: I494bb22c774de5cdf1954bdfb1682e7b40436509
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4236340
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1107454}