Articulate the cancel behavior in the WebTouchEvent.

third_party/WebKit/public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 171 matching lines @@
         IsComposing      = 1 << 14,
 
         AltGrKey         = 1 << 15,
         OSKey            = 1 << 16,
         FnKey            = 1 << 17,
         SymbolKey        = 1 << 18,
 
         ScrollLockOn     = 1 << 19,
     };
 
+    enum DispatchType {

[tdresser, 2016/04/18 19:40:45]

Let's add a comment stating that this is mirrored in platformEvent (and vice
versa). The static_asserts ensure this remains true, but it's nice to have it
stated here.

[dtapuska, 2016/04/18 20:09:09]

Done.

+        Blocking,
+        EventNonBlocking,
+        // All listeners are passive.
+        ListenersNonBlockingPassive,
+        // This value represents a state which would have normally blocking
+        // but was forced to be non-blocking.
+        ListenersForcedNonBlockingPassive,
+    };
+
     // The rail mode for a wheel event specifies the axis on which scrolling is
     // expected to stick. If this axis is set to Free, then scrolling is not
     // stuck to any axis.
     enum RailsMode {
         RailsModeFree       = 0,
         RailsModeHorizontal = 1,
         RailsModeVertical   = 2,
     };
 
     static const int InputModifiers = ShiftKey | ControlKey | AltKey | MetaKey;
@@ skipping 393 matching lines @@
 class WebTouchEvent : public WebInputEvent {
 public:
     // Maximum number of simultaneous touches supported on
     // Ash/Aura.
     enum { touchesLengthCap = 16 };
 
     unsigned touchesLength;
     // List of all touches, regardless of state.
     WebTouchPoint touches[touchesLengthCap];
 
-    // Whether the event can be canceled (with preventDefault). If true then the browser
-    // must wait for an ACK for this event. If false then no ACK IPC is expected.
-    bool cancelable;
+    // Whether the event is blocking, non-blocking, all event
+    // listeners were passive or was forced to be non-blocking.
+    DispatchType dispatchType;
 
     // For a single touch, this is true after the touch-point has moved beyond
     // the platform slop region. For a multitouch, this is true after any
     // touch-point has moved (by whatever amount).
     bool movedBeyondSlopRegion;
 
     // A unique identifier for the touch event.
     uint32_t uniqueTouchEventId;
 
     WebTouchEvent()
         : WebInputEvent(sizeof(WebTouchEvent))
         , touchesLength(0)
-        , cancelable(true)
+        , dispatchType(Blocking)
         , movedBeyondSlopRegion(false)
         , uniqueTouchEventId(0)
     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif