Bug 849404 RTL support for text/plain documents r=dbaron

The addition of unicode-plaintext to the pre element will make RTL text to appear from right to left. The html[dir] pre rule will make sure that this addition won't break the 'switch page direction' functionality.

MozReview-Commit-ID: 2JXJjqt2iuX

--HG--
extra : rebase_source : 98c85228ce50fa145bfaa77051532f69e92ba98d

layout/style/res/plaintext.css

@@ -7,3 +7,8 @@ pre {
   word-wrap: break-word;
   -moz-control-character-visibility: visible;
 }
+
+/* Make text go with the rules of dir=auto, but allow it to be overriden if 'Switch Text Direction' is triggered */
+html:not([dir]) pre {
+  unicode-bidi: plaintext;
+}