[throughput] Fix FrameSequenceLength.Video reporting

We are seeing half of FrameSequenceLength.Video reports length 0.
What's the problem? It turns out that those half are from the main
thread. A video frame is on impl thread only, so main thread gets
0 frames for a video tracker.

This CL fixes it.

Bug: 1112893
Change-Id: I0f6b41625ac2d781dc9c0599e0e8ad28c06a033d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342247Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795949}

cc/metrics/frame_sequence_metrics.cc

@@ -313,6 +313,11 @@ base::Optional<int> FrameSequenceMetrics::ThroughputData::ReportHistogram(
   const auto sequence_type = metrics->type();
   DCHECK_LT(sequence_type, FrameSequenceTrackerType::kMaxType);
 
+  // All video frames are compositor thread only.
+  if (sequence_type == FrameSequenceTrackerType::kVideo &&
+      thread_type == ThreadType::kMain)
+    return base::nullopt;
+
   STATIC_HISTOGRAM_POINTER_GROUP(
       GetFrameSequenceLengthHistogramName(sequence_type),
       static_cast<int>(sequence_type),

cc/metrics/frame_sequence_metrics_unittest.cc

@@ -87,6 +87,19 @@ TEST(FrameSequenceMetricsTest, ScrollingThreadMergeMetrics) {
 }
 #endif  // DCHECK_IS_ON()
 
+TEST(FrameSequenceMetricsTest, VideoReportsOnImplOnly) {
+  base::HistogramTester histograms;
+
+  FrameSequenceMetrics first(FrameSequenceTrackerType::kVideo, nullptr);
+  first.impl_throughput().frames_expected = 120;
+  first.impl_throughput().frames_produced = 80;
+  first.main_throughput().frames_expected = 0;
+  first.main_throughput().frames_produced = 0;
+  first.ReportMetrics();
+  histograms.ExpectTotalCount("Graphics.Smoothness.FrameSequenceLength.Video",
+                              1u);
+}
+
 TEST(FrameSequenceMetricsTest, AllMetricsReported) {
   base::HistogramTester histograms;