Articulate the cancel behavior in the WebTouchEvent.

content/browser/renderer_host/input/input_router_impl.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 "content/browser/renderer_host/input/input_router_impl.h"
 
 #include <math.h>
 
 #include <utility>
 
@@ skipping 352 matching lines @@
 }
 
 void InputRouterImpl::OfferToHandlers(const WebInputEvent& input_event,
                                       const ui::LatencyInfo& latency_info) {
   output_stream_validator_.Validate(input_event);
 
   if (OfferToClient(input_event, latency_info))
     return;
 
   // Touch events should always indicate in the event whether they are
   // cancelable (respect ACK disposition) or not except touchmove.

[tdresser, 2016/04/18 15:12:02]

Can we explain the touchmove case a bit more here?

[dtapuska, 2016/04/18 19:19:33]

Done; moved comments into the web_input_event_traits.

   bool should_block = WebInputEventTraits::ShouldBlockEventStream(input_event);
 
   OfferToRenderer(input_event, latency_info,
                   should_block
                       ? InputEventDispatchType::DISPATCH_TYPE_BLOCKING
                       : InputEventDispatchType::DISPATCH_TYPE_NON_BLOCKING);
 
-  if (WebInputEvent::isTouchEventType(input_event.type) &&
-      input_event.type != WebInputEvent::TouchMove) {
-    const WebTouchEvent& touch = static_cast<const WebTouchEvent&>(input_event);
-    DCHECK_EQ(should_block, touch.cancelable);
-  }
-
   // Generate a synthetic ack if the event was sent so it doesn't block.
   if (!should_block) {
     ProcessInputEventAck(
         input_event.type, INPUT_EVENT_ACK_STATE_IGNORED, latency_info,
         WebInputEventTraits::GetUniqueTouchEventId(input_event),
         IGNORING_DISPOSITION);
   }
 }
 
 bool InputRouterImpl::OfferToClient(const WebInputEvent& input_event,
@@ skipping 246 matching lines @@
 
   flush_requested_ = false;
   client_->DidFlush();
 }
 
 void InputRouterImpl::SetFrameTreeNodeId(int frameTreeNodeId) {
   frame_tree_node_id_ = frameTreeNodeId;
 }
 
 }  // namespace content