Move middle-click autoscroll to synthetic fling.

ui/events/blink/input_handler_proxy.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 1322 matching lines @@
 
   if (disallow_vertical_fling_scroll_ && disallow_horizontal_fling_scroll_)
     fling_is_active = false;
 
   if (fling_is_active) {
     RequestAnimation();
   } else {
     TRACE_EVENT_INSTANT0("input",
                          "InputHandlerProxy::animate::flingOver",
                          TRACE_EVENT_SCOPE_THREAD);
-    CancelCurrentFling();
+    if (fling_parameters_.source_device !=
+        blink::kWebGestureDeviceSyntheticAutoscroll)
+      CancelCurrentFling();
   }
 }
 
 void InputHandlerProxy::MainThreadHasStoppedFlinging() {
   fling_may_be_active_on_main_thread_ = false;
   client_->DidStopFlinging();
 }
 
 void InputHandlerProxy::ReconcileElasticOverscrollAndRootScroll() {
   if (scroll_elasticity_controller_)
@@ skipping 307 matching lines @@
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 void InputHandlerProxy::SetTickClockForTesting(
     std::unique_ptr<base::TickClock> tick_clock) {
   tick_clock_ = std::move(tick_clock);
 }
 
 }  // namespace ui