Implement MouseEvent buttons attribute.

Source/core/page/EventHandler.cpp

Index: Source/core/page/EventHandler.cpp
diff --git a/Source/core/page/EventHandler.cpp b/Source/core/page/EventHandler.cpp
index 3d1279c3b704166d04a0ca9dd72322472a2267e8..84c84a359d3487705b0fb87ed2ee423907837eb1 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -1640,7 +1640,7 @@ bool EventHandler::dispatchDragEvent(const AtomicString& eventType, Node* dragTa
         0, event.globalPosition().x(), event.globalPosition().y(), event.position().x(), event.position().y(),
         event.movementDelta().x(), event.movementDelta().y(),
         event.ctrlKey(), event.altKey(), event.shiftKey(), event.metaKey(),
-        0, nullptr, dataTransfer);
+        0, 0, nullptr, dataTransfer);

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

This looks like a bug too.  DragEvent is a MouseEvent
(https://html.spec.whatwg.org/multipage/interaction.html#the-dragevent-interface),
so technically the current mouse button state should be reflected there too. 
Again, feel free to add a FIXME and worry about this in a follow-up patch (it's
a bit of an edge case).  I'd also be interested to know if FF and IE do this
properly (if neither do, then it's probably a low priority bug for us too).

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

I left a FIXME comment. This seems to be non-trivial.
Also, I've confirmed that FF works correctly. (Sorry about IE, I don't have a
window PC..)

[Rick Byers, 2014/11/18 20:29:18]

Ok.  Thanks for adding the RuntimeEnabledFeature - I think that's a good idea
(it's also a convenient way to be able to disable the feature without reverting
your CLs if we uncover issues).  Hopefully we can flip it to stable very soon
(eg. before Jan 9 branch cut-off for M41).
No problem, comparison to Firefox is useful.  It would be good to compare to one
other major browser too though.  Do you have a Mac (to compare with Safari)?  Or
if you have an Android or iOS device you could use https://remote.modern.ie to
test IE.

 
     dragTarget->dispatchEvent(me.get(), IGNORE_EXCEPTION);
     return me->defaultPrevented();