Bug 811024 part 1: Teach nsCSSFrameConstructor how to split a flex container. r=mats

layout/base/nsCSSFrameConstructor.cpp

@@ -8111,6 +8111,9 @@ nsCSSFrameConstructor::CreateContinuingFrame(nsPresContext* aPresContext,
   } else if (nsGkAtoms::legendFrame == frameType) {
     newFrame = NS_NewLegendFrame(shell, styleContext);
     newFrame->Init(content, aParentFrame, aFrame);
+  } else if (nsGkAtoms::flexContainerFrame == frameType) {
+    newFrame = NS_NewFlexContainerFrame(shell, styleContext);
+    newFrame->Init(content, aParentFrame, aFrame);
   } else {
     NS_RUNTIMEABORT("unexpected frame type");
   }