Implement MouseEvent buttons attribute.

Source/core/events/WheelEvent.cpp

Index: Source/core/events/WheelEvent.cpp
diff --git a/Source/core/events/WheelEvent.cpp b/Source/core/events/WheelEvent.cpp
index 418b5d14772386fff5a649e0dff72ddf6b0f7a03..b2fc119886530ecbc1bf1117e78dfb531a2f628a 100644
--- a/Source/core/events/WheelEvent.cpp
+++ b/Source/core/events/WheelEvent.cpp
@@ -62,8 +62,8 @@ WheelEvent::WheelEvent(const FloatPoint& wheelTicks, const FloatPoint& rawDelta,
     PassRefPtrWillBeRawPtr<AbstractView> view, const IntPoint& screenLocation, const IntPoint& pageLocation,
     bool ctrlKey, bool altKey, bool shiftKey, bool metaKey)
     : MouseEvent(EventTypeNames::wheel, true, true, view, 0, screenLocation.x(), screenLocation.y(),
-        pageLocation.x(), pageLocation.y(), 0, 0, ctrlKey, altKey, shiftKey, metaKey, 0, nullptr,
-        nullptr, false, PlatformMouseEvent::RealOrIndistinguishable)
+        pageLocation.x(), pageLocation.y(), 0, 0, ctrlKey, altKey, shiftKey, metaKey, 0, 0,

[Rick Byers, 2014/11/14 19:48:09]

WheelEvent is a MouseEvent too, so I think you'll need to support and test the
buttons value on it too (although I'd be curious to hear whether FireFox and IE
set it properly for wheel).

If that's going to take some non-trivial work (eg. maybe chromium isn't setting
the modifiers on WebWheelEvent today), then feel free to add a FIXME here
referencing your bug# and worry about that in follow-up CLs.  We don't want to
leave the API partially implemented for long though, we can put it behind a
RuntimeEnabledFeature if it's likely to take more than a week or so.

[zino, 2014/11/18 14:19:35]

Done.

+        nullptr, nullptr, false, PlatformMouseEvent::RealOrIndistinguishable)
     , m_wheelDelta(wheelTicks.x() * TickMultiplier, wheelTicks.y() * TickMultiplier)
     , m_deltaX(-rawDelta.x())
     , m_deltaY(-rawDelta.y())