Enumerate the return value of dispatchEvent so it is clear.

third_party/WebKit/Source/core/events/EventTarget.h

Index: third_party/WebKit/Source/core/events/EventTarget.h
diff --git a/third_party/WebKit/Source/core/events/EventTarget.h b/third_party/WebKit/Source/core/events/EventTarget.h
index 5f56ecb04adcf376f578f59ca61a2f76aede39e3..e59912d21f570b2663f7a28826efd8e201ffe422 100644
--- a/third_party/WebKit/Source/core/events/EventTarget.h
+++ b/third_party/WebKit/Source/core/events/EventTarget.h
@@ -79,6 +79,17 @@ public:
     OwnPtr<FiringEventIteratorVector> firingEventIterators;
 };
 
+enum class DispatchEventResult {
+    // Event was not consumed by application or system.

[philipj_slow, 2016/02/24 10:19:51]

s/consumed/canceled/, s/application/event handler/ and s/system/default event
handler/ would be more familiar terminology.

[dtapuska, 2016/02/24 22:14:16]

Done.

+    NotCanceled,
+    // Event was consumed by application itself; ie. a script handler calling preventDefault.
+    CanceledByEventHandler,
+    // Event was consumed by the system; ie. executing the default action.
+    CanceledByDefaultEventHandler,

[philipj_slow, 2016/02/24 10:19:51]

Add stern warnings that these two should not be used?

[dtapuska, 2016/02/24 22:14:16]

Done.

+    // Event was consumed but suppressed before dispatched to application.
+    CanceledBeforeDispatch,
+};
+
 // This is the base class for all DOM event targets. To make your class an
 // EventTarget, follow these steps:
 // - Make your IDL interface inherit from EventTarget.
@@ -138,7 +149,7 @@ public:
     bool removeEventListener(const AtomicString& eventType, PassRefPtrWillBeRawPtr<EventListener>, EventListenerOptions&);
     virtual void removeAllEventListeners();
 
-    bool dispatchEvent(PassRefPtrWillBeRawPtr<Event>);
+    DispatchEventResult dispatchEvent(PassRefPtrWillBeRawPtr<Event>);
 
     // dispatchEventForBindings is intended to only be called from
     // javascript originated calls. This method will validate and may adjust
@@ -156,7 +167,7 @@ public:
     EventListenerVector* getEventListeners(const AtomicString& eventType);
     Vector<AtomicString> eventTypes();
 
-    bool fireEventListeners(Event*);
+    DispatchEventResult fireEventListeners(Event*);
 
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 
@@ -167,7 +178,7 @@ protected:
 
     virtual bool addEventListenerInternal(const AtomicString& eventType, PassRefPtrWillBeRawPtr<EventListener>, const EventListenerOptions&);
     virtual bool removeEventListenerInternal(const AtomicString& eventType, PassRefPtrWillBeRawPtr<EventListener>, const EventListenerOptions&);
-    virtual bool dispatchEventInternal(PassRefPtrWillBeRawPtr<Event>);
+    virtual DispatchEventResult dispatchEventInternal(PassRefPtrWillBeRawPtr<Event>);
 
     // Subclasses should likely not override these themselves; instead, they should subclass EventTargetWithInlineData.
     virtual EventTargetData* eventTargetData() = 0;