Bug 1125963 - Part 1: Fix serialization of the pre-wrap elements that Thunderbird relies on; r=bzbarsky

This ensures that the plaintext serializer doesn't use the preformatted
text code path if we have encountered a pre-wrap element that
Thunderbird uses (which means setting white-space: pre-wrap and width:
NNch on the body element.)

It also ensures that we use 0 as the wrap column number passed down to
the plaintext serializer, instead of -1, which this code seems to be
unable to handle properly.

dom/html/nsTextEditorState.cpp

@@ -1713,7 +1713,7 @@ nsTextEditorState::InitializeRootNode()
   nsAutoString classValue;
   classValue.AppendLiteral("anonymous-div");
   int32_t wrapCols = GetWrapCols();
-  if (wrapCols >= 0) {
+  if (wrapCols > 0) {
     classValue.AppendLiteral(" wrap");
   }
   if (!IsSingleLineTextControl()) {