Use Blink's flag to prevent fling boosting.

This flag prevents the flingCancel event from being used
as a part of a fling boost event sequence.
The synthetic FlingCancel method works properly now and cancels flings
instantly, without deferring flingCancel event.

See: https://codereview.chromium.org/789223002

BUG=440886

Review URL: https://codereview.chromium.org/792833002

Cr-Commit-Position: refs/heads/master@{#308599}

content/browser/android/content_view_core_impl.cc

@@ -1036,6 +1036,8 @@ void ContentViewCoreImpl::FlingStart(JNIEnv* env, jobject obj, jlong time_ms,
 void ContentViewCoreImpl::FlingCancel(JNIEnv* env, jobject obj, jlong time_ms) {
   WebGestureEvent event = MakeGestureEvent(
       WebInputEvent::GestureFlingCancel, time_ms, 0, 0);
+  event.data.flingCancel.preventBoosting = true;
+
   SendGestureEvent(event);
 }
 

content/renderer/input/input_handler_proxy.cc

@@ -567,6 +567,9 @@ bool InputHandlerProxy::FilterInputEventForFlingBoosting(
   const WebGestureEvent& gesture_event =
       static_cast<const WebGestureEvent&>(event);
   if (gesture_event.type == WebInputEvent::GestureFlingCancel) {
+    if (gesture_event.data.flingCancel.preventBoosting)
+      return false;
+
     if (current_fling_velocity_.LengthSquared() < kMinBoostFlingSpeedSquare)
       return false;
 

content/renderer/input/input_handler_proxy_unittest.cc

@@ -1954,6 +1954,26 @@ TEST_F(InputHandlerProxyTest, NoFlingBoostIfFlingTooSlow) {
   VERIFY_AND_RESET_MOCKS();
 }
 
+TEST_F(InputHandlerProxyTest, NoFlingBoostIfPreventBoostingFlagIsSet) {
+  base::TimeDelta dt = base::TimeDelta::FromMilliseconds(10);
+  base::TimeTicks time = base::TimeTicks() + dt;
+  WebFloatPoint fling_delta = WebFloatPoint(1000, 0);
+  WebPoint fling_point = WebPoint(7, 13);
+
+  StartFling(
+      time, blink::WebGestureDeviceTouchscreen, fling_delta, fling_point);
+
+  EXPECT_CALL(mock_input_handler_, ScrollEnd());
+
+  // Cancel the fling. The fling cancellation should not be deferred because of
+  // prevent boosting flag set.
+  gesture_.data.flingCancel.preventBoosting = true;
+  time += dt;
+  CancelFling(time);
+
+  // VERIFY_AND_RESET_MOCKS already called by CancelFling
+}
+
 TEST_F(InputHandlerProxyTest, FlingBoostTerminatedDuringScrollSequence) {
   base::TimeDelta dt = base::TimeDelta::FromMilliseconds(10);
   base::TimeTicks time = base::TimeTicks() + dt;