Articulate the cancel behavior in the WebTouchEvent.

third_party/WebKit/public/web/WebInputEvent.h

Index: third_party/WebKit/public/web/WebInputEvent.h
diff --git a/third_party/WebKit/public/web/WebInputEvent.h b/third_party/WebKit/public/web/WebInputEvent.h
index d2e24f8f1e33fe979d8ea99fdb3fba8873e4faaf..71187490df969f5d1d846f832ae80bf144ba54ce 100644
--- a/third_party/WebKit/public/web/WebInputEvent.h
+++ b/third_party/WebKit/public/web/WebInputEvent.h
@@ -189,6 +189,16 @@ public:
         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.
@@ -602,9 +612,9 @@ public:
     // 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
@@ -617,7 +627,7 @@ public:
     WebTouchEvent()
         : WebInputEvent(sizeof(WebTouchEvent))
         , touchesLength(0)
-        , cancelable(true)
+        , dispatchType(Blocking)
         , movedBeyondSlopRegion(false)
         , uniqueTouchEventId(0)
     {