Bug 1298318 followup - Add notice comment in SVGImageContext. r=dholbert

Comment-only, so DONTBUILD (and KWierso says it's OK to land this on CLOSED TREE)

MozReview-Commit-ID: BKxFHeXxipf

layout/svg/SVGImageContext.h

@@ -76,6 +76,7 @@ private:
     return aPAR.Hash();
   }
 
+  // NOTE: When adding new member-vars, remember to update Hash() & operator==.
   CSSIntSize                    mViewportSize;
   Maybe<SVGPreserveAspectRatio> mPreserveAspectRatio;
   gfxFloat                      mGlobalOpacity;