scheduler: Reset pending input event count on navigation

When a navigation happens, we should reset the number of pending input
events in the user model to avoid waiting for an acknowledgement which
may never arrive.

This is a speculative mitigation for crbug.com/652294.

BUG=652294

Review-Url: https://codereview.chromium.org/2476263002
Cr-Commit-Position: refs/heads/master@{#430281}

third_party/WebKit/Source/platform/scheduler/renderer/user_model.cc

@@ -196,6 +196,7 @@ void UserModel::Reset(base::TimeTicks now) {
   last_reset_time_ = now;
   is_gesture_active_ = false;
   is_gesture_expected_ = false;
+  pending_input_event_count_ = 0;
 }
 
 void UserModel::AsValueInto(base::trace_event::TracedValue* state) const {

third_party/WebKit/Source/platform/scheduler/renderer/user_model_unittest.cc

@@ -252,5 +252,15 @@ TEST_F(UserModelTest, IsGestureExpectedToContinue_LongAfterGestureStarted) {
   EXPECT_EQ(base::TimeDelta(), prediction_valid_duration);
 }
 
+TEST_F(UserModelTest, ResetPendingInputCount) {
+  user_model_->DidStartProcessingInputEvent(
+      blink::WebInputEvent::Type::GestureScrollBegin, clock_->NowTicks());
+  EXPECT_EQ(priority_escalation_after_input_duration(),
+            user_model_->TimeLeftInUserGesture(clock_->NowTicks()));
+  user_model_->Reset(clock_->NowTicks());
+  EXPECT_EQ(base::TimeDelta(),
+            user_model_->TimeLeftInUserGesture(clock_->NowTicks()));
+}
+
 }  // namespace scheduler
 }  // namespace blink