Populates sourceDevice attribute into MouseEvent

Source/core/events/MouseEvent.idl

Index: Source/core/events/MouseEvent.idl
diff --git a/Source/core/events/MouseEvent.idl b/Source/core/events/MouseEvent.idl
index 21c0a2fd5059112c134a1d412a885c598ed4e266..49b3f50ef770a903405101ee9e2ec3c3189a5b59 100644
--- a/Source/core/events/MouseEvent.idl
+++ b/Source/core/events/MouseEvent.idl
@@ -53,6 +53,23 @@
                                        [Default=Undefined] optional boolean metaKey,
                                        [Default=Undefined] optional unsigned short button,
                                        [Default=Undefined] optional EventTarget? relatedTarget);
+                                       
+    [RuntimeEnabled=InputDevice, CallWith=ScriptState] void initMouseEvent([Default=Undefined] optional DOMString type,
+                                       [Default=Undefined] optional boolean bubbles,
+                                       [Default=Undefined] optional boolean cancelable,
+                                       [Default=Undefined] optional Window? view,
+                                       [Default=Undefined] optional long detail,
+                                       [Default=Undefined] optional long screenX,
+                                       [Default=Undefined] optional long screenY,
+                                       [Default=Undefined] optional long clientX,
+                                       [Default=Undefined] optional long clientY,
+                                       [Default=Undefined] optional boolean ctrlKey,
+                                       [Default=Undefined] optional boolean altKey,
+                                       [Default=Undefined] optional boolean shiftKey,
+                                       [Default=Undefined] optional boolean metaKey,
+                                       [Default=Undefined] optional unsigned short button,
+                                       [Default=Undefined] optional EventTarget? relatedTarget,
+                                       [Default=Undefined] InputDevice? sourceDevice);                                   

[Rick Byers, 2015/06/23 00:55:57]

Any chance the bindings generator is smart enough to let you add this as
'optional InputDevice? ...' on the above method, even though InputDevice is
RuntimeEnabled?  That would be simpler if it works.  Otherwise temporarily
adding another overload is fine, and we can remove it once InputDevice is
enabled by default.

 
     // Pointer Lock
     // https://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html#extensions-to-the-mouseevent-interface