Perfetto: Enable filtering JSON output based on label

Tracing clients like devtools will request trace events only
for a specific label, like "traceEvents", and the expected output
is then just a flat JSON list of corresponding events, with no
extra metadata or data from other labels.

R=ssid@chromium.org,eseckler@chromium.org

Bug: 839084
Change-Id: I39774f9e8798baee64d4416eac2ba67cf734a3f8
Reviewed-on: https://chromium-review.googlesource.com/c/1481122
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634592}
4 files changed