Bug 1097128 - Enable compile-time support for vertical writing modes. r=smontagu

layout/generic/WritingModes.h

@@ -16,7 +16,12 @@
 // of transitioning layout to use writing-mode and logical directions, but
 // not yet ready to ship vertical support.
 
-/* #define WRITING_MODE_VERTICAL_ENABLED 1 */
+// XXX To be removed, and the #ifdef blocks below made unconditional,
+//     once we're confident we can leave it permanently enabled.
+
+#ifndef RELEASE_BUILD
+#define WRITING_MODE_VERTICAL_ENABLED 1
+#endif
 
 // It is the caller's responsibility to operate on logical-coordinate objects
 // with matched writing modes. Failure to do so will be a runtime bug; the