Bug 1543501 - Add operator<< to ScaleFactor. r=kats

This allows it to be used more easily with MOZ_DBG().

Differential Revision: https://phabricator.services.mozilla.com/D27000

--HG--
extra : moz-landing-system : lando

gfx/2d/ScaleFactor.h

@@ -7,6 +7,8 @@
 #ifndef MOZILLA_GFX_SCALEFACTOR_H_
 #define MOZILLA_GFX_SCALEFACTOR_H_
 
+#include <ostream>
+
 #include "mozilla/Attributes.h"
 
 #include "gfxPoint.h"
@@ -81,6 +83,11 @@ struct ScaleFactor {
       const ScaleFactor<other, src>& aOther) const {
     return ScaleFactor<other, dst>(scale * aOther.scale);
   }
+
+  friend std::ostream& operator<<(std::ostream& aStream,
+                                  const ScaleFactor<src, dst>& aSF) {
+    return aStream << aSF.scale;
+  }
 };
 
 }  // namespace gfx