Background tracing: Disable when highres clock is not available

R=simonhatch
BUG=548756

Review URL: https://codereview.chromium.org/1408243009

Cr-Commit-Position: refs/heads/master@{#357160}

content/browser/tracing/background_tracing_manager_impl.cc

@@ -39,6 +39,7 @@ enum BackgroundTracingMetrics {
   FINALIZATION_DISALLOWED = 6,
   FINALIZATION_STARTED = 7,
   FINALIZATION_COMPLETE = 8,
+  SCENARIO_ACTION_FAILED_LOWRES_CLOCK = 9,
   NUMBER_OF_BACKGROUND_TRACING_METRICS,
 };
 
@@ -134,6 +135,13 @@ bool BackgroundTracingManagerImpl::SetActiveScenario(
   if (is_tracing_)
     return false;
 
+  // If we don't have a high resolution timer available, traces will be
+  // too inaccurate to be useful.
+  if (!base::TimeTicks::IsHighResolution()) {
+    RecordBackgroundTracingMetric(SCENARIO_ACTION_FAILED_LOWRES_CLOCK);
+    return false;
+  }
+
   bool requires_anonymized_data = (data_filtering == ANONYMIZE_DATA);
 
   // If the I/O thread isn't running, this is a startup scenario and

tools/metrics/histograms/histograms.xml

@@ -53714,6 +53714,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="6" label="Trace finalization disallowed"/>
   <int value="7" label="Trace finalization started"/>
   <int value="8" label="Trace finalization complete"/>
+  <int value="9" label="Scenario activation failed due to lowres clock"/>
 </enum>
 
 <enum name="BackingStoreResults" type="int">