Articulate the cancel behavior in the WebTouchEvent.

content/common/input/web_input_event_traits.cc

Index: content/common/input/web_input_event_traits.cc
diff --git a/content/common/input/web_input_event_traits.cc b/content/common/input/web_input_event_traits.cc
index a25cf8ed2ba8a60f70832d3e410d70f65f18d35c..6c4c8b523477a151079dadde8f68eff08e969d99 100644
--- a/content/common/input/web_input_event_traits.cc
+++ b/content/common/input/web_input_event_traits.cc
@@ -111,9 +111,9 @@ void ApppendTouchPointDetails(const WebTouchPoint& point, std::string* result) {
 
 void ApppendEventDetails(const WebTouchEvent& event, std::string* result) {
   StringAppendF(result,
-                "{\n Touches: %u, Cancelable: %d, CausesScrolling: %d,"
+                "{\n Touches: %u, DispatchType: %d, CausesScrolling: %d,"
                 " uniqueTouchEventId: %u\n[\n",
-                event.touchesLength, event.cancelable,
+                event.touchesLength, event.dispatchType,
                 event.movedBeyondSlopRegion, event.uniqueTouchEventId);
   for (unsigned i = 0; i < event.touchesLength; ++i)
     ApppendTouchPointDetails(event.touches[i], result);
@@ -481,11 +481,24 @@ bool WebInputEventTraits::ShouldBlockEventStream(const WebInputEvent& event) {
     case WebInputEvent::GestureTapCancel:
     case WebInputEvent::GesturePinchBegin:
     case WebInputEvent::GesturePinchEnd:
+      return false;
+
+    // Touch cancel should always be non-blocking.
     case WebInputEvent::TouchCancel:
+      DCHECK_NE(WebInputEvent::Blocking,
+                static_cast<const WebTouchEvent&>(event).dispatchType);
       return false;
+
+    // Touch start and touch end indicate whether they are non-blocking
+    // (aka uncancelable) on the event.
     case WebInputEvent::TouchStart:
     case WebInputEvent::TouchEnd:
-      return static_cast<const WebTouchEvent&>(event).cancelable;
+      return static_cast<const WebTouchEvent&>(event).dispatchType ==
+             WebInputEvent::Blocking;
+
+    // Touch move events may be non-blocking but are always explicitly
+    // acknowledge by the renderer so they block the event stream.
+    case WebInputEvent::TouchMove:
     default:
       return true;
   }