Bug 1361949 - Add missing 'using namespace::widget' to widget/windows/nsDragService.cpp r=jwatt

Because ModifierKeyState is in the 'mozilla::widget' namespace, this file uses
it without qualification.

MozReview-Commit-ID: H6t3AqLjRwJ

--HG--
extra : rebase_source : 69b788761ee332cc94acc4be6ae6ce0bb50f0e98

widget/windows/nsDragService.cpp

@@ -45,6 +45,7 @@
 
 using namespace mozilla;
 using namespace mozilla::gfx;
+using namespace mozilla::widget;
 
 //-------------------------------------------------------------------------
 //