Bug 680085 - Don't traverse dirty text frames when computing rendered text. r=roc

We now bail in GetRenderedText when we encounter dirty text frames. This should be okay since dirty text frames should be reflowed at some point and we'll refresh our accessible text cache. (relanding)

layout/generic/nsTextFrameThebes.cpp

@@ -7608,6 +7608,11 @@ nsresult nsTextFrame::GetRenderedText(nsAString* aAppendToString,
        textFrame = static_cast<nsTextFrame*>(textFrame->GetNextContinuation())) {
     // For each text frame continuation in this block ...
 
+    if (textFrame->GetStateBits() & NS_FRAME_IS_DIRTY) {
+      // We don't trust dirty frames, expecially when computing rendered text.
+      break;
+    }
+
     // Ensure the text run and grab the gfxSkipCharsIterator for it
     gfxSkipCharsIterator iter = textFrame->EnsureTextRun();
     if (!textFrame->mTextRun)