Bug 1203472 - [css-grid] Add missing |mIsSubgrid| check. r=simon.sapin@exyr.org

The missing |mIsSubgrid| check caused nsStylePosition::CalcDifference
to return zero even after the property value was changed to 'subgrid'.
See bug 1176782 comment 14 (and the comments leading up to that) for
details.

layout/style/nsStyleStruct.h

@@ -1283,7 +1283,8 @@ struct nsStyleGridTemplate {
   }
 
   inline bool operator!=(const nsStyleGridTemplate& aOther) const {
-    return mLineNameLists != aOther.mLineNameLists ||
+    return mIsSubgrid != aOther.mIsSubgrid ||
+           mLineNameLists != aOther.mLineNameLists ||
            mMinTrackSizingFunctions != aOther.mMinTrackSizingFunctions ||
            mMaxTrackSizingFunctions != aOther.mMaxTrackSizingFunctions;
   }