Add telemetry to track the time an event spent waiting for the main thread.

tools/perf/metrics/smoothness_unittest.py

Index: tools/perf/metrics/smoothness_unittest.py
diff --git a/tools/perf/metrics/smoothness_unittest.py b/tools/perf/metrics/smoothness_unittest.py
index 2a42f9dd24e39b71792dc1934b87ab0ed74ed406..42cf49a116f5123202860f61140a5b5e6345dd21 100644
--- a/tools/perf/metrics/smoothness_unittest.py
+++ b/tools/perf/metrics/smoothness_unittest.py
@@ -37,7 +37,11 @@ class SmoothnessMetricsUnitTest(unittest.TestCase):
         'touchAckedCount': 190,
         'totalTouchAckedLatency': 1.9,
         'scrollUpdateCount': 200,
-        'totalScrollUpdateLatency': 2.0}
+        'totalScrollUpdateLatency': 2.0,
+        'implToMainTouchEventCount': 100,
+        'totalImplToMainTouchEventLatency': 0.7,
+        'implToMainGestureScrollEventCount': 133,
+        'totalImplToMainGestureScrollEventLatency': 2.7}
     stats = GpuRenderingStats(mock_rendering_stats_deltas)
 
     res = PageMeasurementResults()