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,
     };
 
+    // These values are direct mappings of the values in PlatformEvent

[Rick Byers, 2016/04/20 15:30:26]

nit: add a comment above this describing briefly the purpose of this enum.  I.e.
"indicates whether the browser needs to block on the ACK result for this event,
and if not why not (for metrics/diagnostics purposes)".

[dtapuska, 2016/04/20 19:38:11]

Done.

+    // so the values can be cast between the enumerations. static_asserts
+    // checking this are in web/WebInputEventConversion.cpp.
+    enum DispatchType {
+        Blocking,

[Rick Byers, 2016/04/20 15:30:26]

nit: add comment saying "i.e. can be canceled"

[dtapuska, 2016/04/20 19:38:12]

Done.

+        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