Plumb smooth scrolling in Chromium compositor

content/browser/android/content_view_core_impl.cc

Index: content/browser/android/content_view_core_impl.cc
diff --git a/content/browser/android/content_view_core_impl.cc b/content/browser/android/content_view_core_impl.cc
index dd9bab868ac8c7921370b381c6b292f95a86996b..8e317fd53403bf80fb526ad7721a31a097c0734e 100644
--- a/content/browser/android/content_view_core_impl.cc
+++ b/content/browser/android/content_view_core_impl.cc
@@ -1066,6 +1066,31 @@ void ContentViewCoreImpl::FlingCancel(JNIEnv* env, jobject obj, jlong time_ms) {
   SendGestureEvent(event);
 }
 
+void ContentViewCoreImpl::SmoothScrollStart(JNIEnv* env,
+                                            jobject obj,
+                                            jlong time_ms,
+                                            float start_x,
+                                            float start_y,
+                                            float dx,
+                                            float dy,
+                                            jlong duration_ms) {
+  WebGestureEvent event = MakeGestureEvent(WebInputEvent::GestureFlingStart,
+                                           time_ms, start_x, start_y);
+  event.data.flingStart.isSmoothScroll = true;
+  event.data.flingStart.dx = dx / dpi_scale();
+  event.data.flingStart.dy = dy / dpi_scale();
+  event.data.flingStart.durationMs = duration_ms;
+  event.data.flingStart.targetViewport = true;
+
+  // Velocities are not used for computing the smooth scroll animation curve.
+  // But they are included here anyways because zero-velocity fling results in
+  // GestureScrollEnd immediately.
+  event.data.flingStart.velocityX = dx / dpi_scale() / time_ms * 1000;
+  event.data.flingStart.velocityY = dy / dpi_scale() / time_ms * 1000;
+
+  SendGestureEvent(event);
+}
+
 void ContentViewCoreImpl::SingleTap(JNIEnv* env, jobject obj, jlong time_ms,
                                     jfloat x, jfloat y) {
   // Tap gestures should always be preceded by a TapDown, ensuring consistency