Reland of Trace the stats report as JSON instead of each stat separately. (patchset #1 id:1 of https://codereview.webrtc.org/3001683002/ )

Reason for revert:
Fixed

Original issue's description:
> Revert of Trace the stats report as JSON instead of each stat separately. (patchset #3 id:100001 of https://codereview.webrtc.org/2986453002/ )
>
> Reason for revert:
> It breaks a downstream project.
>
> Original issue's description:
> > Trace the stats report as JSON instead of each stat separately.
> >
> > Trace the whole report as a string instead of each field on it's own. And test that the traces collected are valid.
> >
> > R=tommi@webrtc.org, hbos@webrtc.org
> > BUG=chromium:653087
> >
> > Review-Url: https://codereview.webrtc.org/2986453002
> > Cr-Commit-Position: refs/heads/master@{#19341}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/80c65780e618f90369295fe5daf8075e9b374c26
>
> TBR=hbos@webrtc.org,tommi@webrtc.org,ehmaldonado@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=chromium:653087
>
> Review-Url: https://codereview.webrtc.org/3001683002
> Cr-Commit-Position: refs/heads/master@{#19344}
> Committed: https://chromium.googlesource.com/external/webrtc/+/3439c893585e0958502ef397056a372be1777688

BUG=chromium:653087

Review-Url: https://codereview.webrtc.org/3000943002
Cr-Original-Commit-Position: refs/heads/master@{#19673}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8ab0fd80ad4af302d8f72e94e577458084772288
2 files changed