Populates sourceDevice attribute into MouseEvent

Source/core/events/MouseRelatedEvent.h

Index: Source/core/events/MouseRelatedEvent.h
diff --git a/Source/core/events/MouseRelatedEvent.h b/Source/core/events/MouseRelatedEvent.h
index 6b4ae753e08e1ab1ac95a98cb7c79c0bf42bcb29..36638b112a50ced0b0e8398b2ea0ce0ef6d2c20c 100644
--- a/Source/core/events/MouseRelatedEvent.h
+++ b/Source/core/events/MouseRelatedEvent.h
@@ -65,10 +65,12 @@ namespace blink {
 
     protected:
         MouseRelatedEvent();
+        // TODO(lanwei): Will make this argument non-optional and all the callers need to provide
+        // sourceDevice even when it is null, see https://crbug.com/476530.
         MouseRelatedEvent(const AtomicString& type, bool canBubble, bool cancelable,
             PassRefPtrWillBeRawPtr<AbstractView>, int detail, const IntPoint& screenLocation,
             const IntPoint& rootFrameLocation, const IntPoint& movementDelta, bool ctrlKey, bool altKey,
-            bool shiftKey, bool metaKey, bool isSimulated = false);
+            bool shiftKey, bool metaKey, bool isSimulated = false, InputDevice* sourceDevice = nullptr);
 
         void initCoordinates();
         void initCoordinates(const LayoutPoint& clientLocation);