diff -r 000000000000 -r 6474c204b198 layout/base/PaintTracker.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/base/PaintTracker.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,34 @@
+/* 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_PaintTracker_h
+#define mozilla_PaintTracker_h
+
+#include "mozilla/Attributes.h"
+#include "nsDebug.h"
+
+namespace mozilla {
+
+class MOZ_STACK_CLASS PaintTracker
+{
+public:
+  PaintTracker() {
+    ++gPaintTracker;
+  }
+  ~PaintTracker() {
+    NS_ASSERTION(gPaintTracker > 0, "Mismatched constructor/destructor");
+    --gPaintTracker;
+  }
+
+  static bool IsPainting() {
+    return !!gPaintTracker;
+  }
+
+private:
+  static int gPaintTracker;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_PaintTracker_h