b=655451 record first of math frame continuations as mstyle ancestor r=bz

--HG--
extra : transplant_source : sg%609%81c%AE%8A%E0%F9%DF9M%BEn%B7%19%90%B4%10

layout/mathml/nsMathMLFrame.cpp

@@ -219,7 +219,7 @@ nsMathMLFrame::GetPresentationDataFrom(nsIFrame*           aFrame,
       if (display->mDisplay == NS_STYLE_DISPLAY_BLOCK) {
         aPresentationData.flags |= NS_MATHML_DISPLAYSTYLE;
       }
-      aPresentationData.mstyle = frame;
+      aPresentationData.mstyle = frame->GetFirstContinuation();
       break;
     }
     frame = frame->GetParent();