[tools/perf] Correctly update the tracing status in blink_perf.py

Bug: 1042644
Change-Id: Ib7e935618641a13999bf045646ac6a94682f5b1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003858Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Cr-Commit-Position: refs/heads/master@{#732509}

tools/perf/benchmarks/blink_perf.py

@@ -321,6 +321,7 @@ class _BlinkPerfMeasurement(legacy_page_test.LegacyPageTest):
     if self._is_tracing:
       trace_data = tab.browser.platform.tracing_controller.StopTracing()
       results.AddTraces(trace_data)
+      self._is_tracing = False
 
       trace_events_to_measure = tab.EvaluateJavaScript(
           'window.testRunner.traceEventsToMeasure')