--- a/gfx/2d/2D.h
+++ b/gfx/2d/2D.h
@@ -21,16 +21,17 @@
 // to be able to hold on to a GLContext.
 #include "mozilla/GenericRefCounted.h"
 #include "mozilla/MemoryReporting.h"
 
 // This RefPtr class isn't ideal for usage in Azure, as it doesn't allow T**
 // outparams using the &-operator. But it will have to do as there's no easy
 // solution.
 #include "mozilla/RefPtr.h"
+#include "mozilla/ServoUtils.h"
 #include "mozilla/WeakPtr.h"
 
 #include "mozilla/DebugOnly.h"
 
 #ifdef MOZ_ENABLE_FREETYPE
 #include <string>
 #endif
 
@@ -58,16 +59,33 @@ class SkCanvas;
 struct gfxFontStyle;
 
 struct CGContext;
 typedef struct CGContext *CGContextRef;
 
 namespace mozilla {
 
 namespace gfx {
+class UnscaledFont;
+}
+
+template<>
+struct WeakPtrTraits<gfx::UnscaledFont>
+{
+  static void AssertSafeToAccessFromNonOwningThread()
+  {
+    // We want to allow UnscaledFont objects that were created on the main
+    // thread to be accessed from other threads if the Servo font metrics
+    // mutex is locked, and for objects created on Servo style worker threads
+    // to be accessed later back on the main thread.
+    AssertIsMainThreadOrServoFontMetricsLocked();
+  }
+};
+
+namespace gfx {
 
 class ScaledFont;
 class SourceSurface;
 class DataSourceSurface;
 class DrawTarget;
 class DrawEventRecorder;
 class FilterNode;
 class LogForwarder;