diff -r 000000000000 -r 6474c204b198 content/base/src/nsTextFragmentImpl.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/base/src/nsTextFragmentImpl.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTextFragmentImpl_h__
+#define nsTextFragmentImpl_h__
+
+#include <stdint.h>
+
+template<size_t size> struct Non8BitParameters;
+template<> struct Non8BitParameters<4> {
+  static inline size_t mask() { return 0xff00ff00; }
+  static inline uint32_t alignMask() { return 0x3; }
+  static inline uint32_t numUnicharsPerWord() { return 2; }
+};
+
+template<> struct Non8BitParameters<8> {
+  static inline size_t mask() {
+    static const uint64_t maskAsUint64 = 0xff00ff00ff00ff00ULL;
+    // We have to explicitly cast this 64-bit value to a size_t, or else
+    // compilers for 32-bit platforms will warn about it being too large to fit
+    // in the size_t return type. (Fortunately, this code isn't actually
+    // invoked on 32-bit platforms -- they'll use the <4> specialization above.
+    // So it is, in fact, OK that this value is too large for a 32-bit size_t.)
+    return (size_t)maskAsUint64;
+  }
+  static inline uint32_t alignMask() { return 0x7; }
+  static inline uint32_t numUnicharsPerWord() { return 4; }
+};
+
+#endif