Adjust cancelabilty of touchmoves that are queued when scroll start occurs

content/renderer/input/main_thread_event_queue_task.h

 // Copyright 2017 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.
 
 #ifndef CONTENT_RENDERER_INPUT_MAIN_THREAD_EVENT_QUEUE_TASK_H_
 #define CONTENT_RENDERER_INPUT_MAIN_THREAD_EVENT_QUEUE_TASK_H_
 
 #include "content/common/input/input_event_ack_state.h"
 #include "third_party/WebKit/public/platform/WebInputEvent.h"
 #include "third_party/WebKit/public/platform/scheduler/renderer/renderer_scheduler.h"
 
 namespace content {
 
 class MainThreadEventQueueClient;
 
 // A work item to execute from the main thread event queue.
 // The MainThreadEventQueue supports 2 types of tasks (subclasses):
 // 1) Closures
 // 2) WebInputEvent
 class MainThreadEventQueueTask {
  public:
   virtual ~MainThreadEventQueueTask() {}
 
-  enum class CoalesceResult {
-    Coalesced,
-    CannotCoalesce,
-    // Keep iterating on the queue looking for a matching event with the
-    // same modality.
-    KeepSearching,
+  enum class FilterResult {
+    // The passed in event was coalesced into this event. Don't queue
+    // the new event.
+    CoalescedEvent,
+
+    // Stop invoking FilterNewEvent on any other events in the queue.
+    StopIterating,
+
+    // Keep invoking FilterNewEvent on the next older event in the queue.
+    KeepIterating,
   };
 
-  virtual CoalesceResult CoalesceWith(const MainThreadEventQueueTask&) = 0;
+  // Filter a new event that is about to be queued. Acceptable actions
+  // are to coalesce event, stop iterating or keep iterating.
+  // Iteration of the list begins at the end of the queue (newest to oldest).
+  virtual FilterResult FilterNewEvent(const MainThreadEventQueueTask&) = 0;
   virtual bool IsWebInputEvent() const = 0;
   virtual void Dispatch(int routing_id, MainThreadEventQueueClient*) = 0;
+
   virtual void EventHandled(
       int routing_id,
       blink::scheduler::RendererScheduler* renderer_scheduler,
       MainThreadEventQueueClient* client,
       blink::WebInputEvent::Type type,
       blink::WebInputEventResult result,
       InputEventAckState ack_result) = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_INPUT_MAIN_THREAD_EVENT_QUEUE_TASK_H_