[throughput] Fix a DCHECK crash

Currently we hit a DCHECK in report histogram. The problem is that
for certain type of throughput data (such as CompositorAnimation +
main thread), we do not report it. In this case, we do not reset the
throughput data and it kept accumulated and eventually the number
of expected frames reaches a threshold and caused crash.

This CL fixes the crash and added test.

Bug: 1052848
Change-Id: Ib150872f950f0901a6bce5f10e683e08644c5809
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2061398Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742947}

cc/metrics/frame_sequence_metrics_unittest.cc

@@ -120,6 +120,12 @@ TEST(FrameSequenceMetricsTest, IrrelevantMetricsNotReported) {
       "CompositorAnimation",
       0u);
 
+  // Not reported, but the data should be reset.
+  EXPECT_EQ(first.impl_throughput().frames_expected, 0u);
+  EXPECT_EQ(first.impl_throughput().frames_produced, 0u);
+  EXPECT_EQ(first.main_throughput().frames_expected, 0u);
+  EXPECT_EQ(first.main_throughput().frames_produced, 0u);
+
   FrameSequenceMetrics second(FrameSequenceTrackerType::kRAF, nullptr);
   second.impl_throughput().frames_expected = 120;
   second.impl_throughput().frames_produced = 80;

cc/metrics/frame_sequence_tracker.cc

@@ -232,10 +232,10 @@ void FrameSequenceMetrics::ReportMetrics() {
     frames_checkerboarded_ = 0;
   }
 
-  // Reset the metrics that have already been reported.
-  if (impl_throughput_percent.has_value())
+  // Reset the metrics that reach reporting threshold.
+  if (impl_throughput_.frames_expected >= kMinFramesForThroughputMetric)
     impl_throughput_ = {};
-  if (main_throughput_percent.has_value())
+  if (main_throughput_.frames_expected >= kMinFramesForThroughputMetric)
     main_throughput_ = {};
 }