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The test was flaky a long time ago but now fails consistently.
The TracingStartedInPage trace event doesn't seem to be emitted anymore
so I replaced this with the TracingStartedInBrowser event.

That event doesn't emit nodeIds so I updated the expectation to assert
that they are undfined.

Also a drive-by cleanup to tracing-test which was trying to JSONify
the wrong variable which meant the output in case the test failed
was not helpful.

Also clean up the test itself with const, JS style etc.

Bug: 734762
Change-Id: I3f7f5665ace38525fca5967c5de6f6fba02d3d5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983172Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728177}