bug 1241453 - add an AccessibleOrProxy class r=davidb

We have several places that store either a ProxyAccessible* or an Accessible*
 in the same member using a uintptr_t and stealing the lowest bit of the
 pointer.  The goal of the AccessibleOrProxy class is to make this simpler and
 consolidate the logic involved in doing it.

accessible/base/AccessibleOrProxy.h

@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* 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 mozilla_a11y_AccessibleOrProxy_h
+#define mozilla_a11y_AccessibleOrProxy_h
+
+#include "mozilla/a11y/Accessible.h"
+#include "mozilla/a11y/ProxyAccessible.h"
+
+#include <stdint.h>
+
+namespace mozilla {
+namespace a11y {
+
+/**
+ * This class stores an Accessible* or a ProxyAccessible* in a safe manner
+ * with size sizeof(void*).
+ */
+class AccessibleOrProxy
+{
+public:
+  MOZ_IMPLICIT AccessibleOrProxy(Accessible* aAcc) :
+    mBits(reinterpret_cast<uintptr_t>(aAcc)) {}
+  MOZ_IMPLICIT AccessibleOrProxy(ProxyAccessible* aProxy) :
+    mBits(reinterpret_cast<uintptr_t>(aProxy) | IS_PROXY) {}
+  MOZ_IMPLICIT AccessibleOrProxy(decltype(nullptr)) : mBits(0) {}
+
+  bool IsProxy() const { return mBits & IS_PROXY; }
+  ProxyAccessible* AsProxy() const
+  {
+    if (IsProxy()) {
+      return reinterpret_cast<ProxyAccessible*>(mBits & ~IS_PROXY);
+    }
+
+    return nullptr;
+  }
+
+  bool IsAccessible() const { return !IsProxy(); }
+  Accessible* AsAccessible() const
+  {
+    if (IsAccessible()) {
+      return reinterpret_cast<Accessible*>(mBits);
+    }
+
+    return nullptr;
+  }
+
+  // XXX these are implementation details that ideally would not be exposed.
+  uintptr_t Bits() const { return mBits; }
+  void SetBits(uintptr_t aBits) { mBits = aBits; }
+
+private:
+  uintptr_t mBits;
+  static const uintptr_t IS_PROXY = 0x1;
+};
+
+}
+}
+
+#endif