CANCEL: Make "compositionstart" event cancelable

Source/core/editing/InputMethodController.cpp

Index: Source/core/editing/InputMethodController.cpp
diff --git a/Source/core/editing/InputMethodController.cpp b/Source/core/editing/InputMethodController.cpp
index 47a27e6abba2bbc30475fe03c0fe856aacf7e8ad..eeeb84f330861ff3afd72097a24da9871ca3c9bf 100644
--- a/Source/core/editing/InputMethodController.cpp
+++ b/Source/core/editing/InputMethodController.cpp
@@ -259,7 +259,12 @@ void InputMethodController::setComposition(const String& text, const Vector<Comp
             // We should send a compositionstart event only when the given text is not empty because this
             // function doesn't create a composition node when the text is empty.
             if (!text.isEmpty()) {
-                target->dispatchEvent(CompositionEvent::create(EventTypeNames::compositionstart, m_frame.domWindow(), m_frame.selectedText(), underlines));
+                RefPtrWillBeRawPtr<CompositionEvent> compositionStartEvent = CompositionEvent::create(EventTypeNames::compositionstart, m_frame.domWindow(), m_frame.selectedText(), underlines);
+                if (!target->dispatchEvent(compositionStartEvent)) {
+                    RefPtrWillBeRawPtr<CompositionEvent> compositionEndEvent = CompositionEvent::create(EventTypeNames::compositionend, m_frame.domWindow(), emptyString(), Vector<CompositionUnderline>());
+                    target->dispatchEvent(compositionEndEvent, IGNORE_EXCEPTION);
+                    return;
+                }
                 event = CompositionEvent::create(EventTypeNames::compositionupdate, m_frame.domWindow(), text, underlines);
             }
         } else {