Fix recording of kAnimatingScrollOnMain

We only want to record this reason if it's the only scroll reason (as it
obscures others).

BUG=None

Review-Url: https://codereview.chromium.org/2289593006
Cr-Commit-Position: refs/heads/master@{#415382}

ui/events/blink/input_handler_proxy.cc

@@ -427,6 +427,14 @@ void InputHandlerProxy::RecordMainThreadScrollingReasons(
        ++i) {
     unsigned val = 1 << i;
     if (reasons & val) {
+      if (val == cc::MainThreadScrollingReason::kAnimatingScrollOnMainThread) {
+        // We only want to record "animating scroll on main thread" reason if
+        // it's the only reason. If it's not the only reason, the "real" reason
+        // for scrolling on main is something else, and we only want to pay
+        // attention to that reason.
+        if (reasons & ~val)
+          continue;
+      }
       if (device == blink::WebGestureDeviceTouchscreen) {
         UMA_HISTOGRAM_ENUMERATION(
             kGestureHistogramName, i + 1,

ui/events/blink/input_handler_proxy_unittest.cc

@@ -2782,6 +2782,20 @@ TEST_P(InputHandlerProxyTest, MainThreadScrollingMouseWheelHistograms) {
           cc::MainThreadScrollingReason::kPageOverlay |
           cc::MainThreadScrollingReason::kAnimatingScrollOnMainThread);
 
+  EXPECT_THAT(
+      histogram_tester().GetAllSamples("Renderer4.MainThreadWheelScrollReason"),
+      testing::ElementsAre(base::Bucket(1, 1), base::Bucket(3, 1),
+                           base::Bucket(5, 1)));
+
+  // We only want to record "animating scroll on main thread" reason if it's the
+  // only reason. If it's not the only reason, the "real" reason for scrolling
+  // on main is something else, and we only want to pay attention to that
+  // reason. So we should only include this reason in the histogram when its on
+  // its own.
+  input_handler_->RecordMainThreadScrollingReasonsForTest(
+      blink::WebGestureDeviceTouchpad,
+      cc::MainThreadScrollingReason::kAnimatingScrollOnMainThread);
+
   EXPECT_THAT(
       histogram_tester().GetAllSamples("Renderer4.MainThreadWheelScrollReason"),
       testing::ElementsAre(base::Bucket(1, 1), base::Bucket(3, 1),