Implement MouseEvent buttons attribute.

Source/web/tests/WebFrameTest.cpp

Index: Source/web/tests/WebFrameTest.cpp
diff --git a/Source/web/tests/WebFrameTest.cpp b/Source/web/tests/WebFrameTest.cpp
index c3ca962fc970122224ed2f9c931dc5ecfa5caaf7..7ad752800491db6273b1f7e0c9d3adc4df63c4a7 100644
--- a/Source/web/tests/WebFrameTest.cpp
+++ b/Source/web/tests/WebFrameTest.cpp
@@ -5506,7 +5506,7 @@ TEST_F(WebFrameTest, SimulateFragmentAnchorMiddleClick)
     destination.setFragmentIdentifier("test");
 
     RefPtrWillBeRawPtr<Event> event = MouseEvent::create(EventTypeNames::click, false, false,
-        document->domWindow(), 0, 0, 0, 0, 0, 0, 0, false, false, false, false, 1, nullptr, nullptr);
+        document->domWindow(), 0, 0, 0, 0, 0, 0, 0, false, false, false, false, 1, 0, nullptr, nullptr);
     FrameLoadRequest frameRequest(document, ResourceRequest(destination));
     frameRequest.setTriggeringEvent(event);
     toLocalFrame(webViewHelper.webViewImpl()->page()->mainFrame())->loader().load(frameRequest);
@@ -5555,7 +5555,7 @@ TEST_F(WebFrameTest, ModifiedClickNewWindow)
 
     // ctrl+click event
     RefPtrWillBeRawPtr<Event> event = MouseEvent::create(EventTypeNames::click, false, false,
-        document->domWindow(), 0, 0, 0, 0, 0, 0, 0, true, false, false, false, 0, nullptr, nullptr);
+        document->domWindow(), 0, 0, 0, 0, 0, 0, 0, true, false, false, false, 0, 0, nullptr, nullptr);
     FrameLoadRequest frameRequest(document, ResourceRequest(destination));
     frameRequest.setTriggeringEvent(event);
     UserGestureIndicator gesture(DefinitelyProcessingUserGesture);