Bug 855790 - Fix Clang trunk compilation error on gfxPlatformMac.h r=mattwoodrow a=overholt

gfx/thebes/gfxPlatformMac.h

@@ -18,7 +18,8 @@
 
 class gfxTextRun;
 class gfxFontFamily;
-class mozilla::gfx::DrawTarget;
+
+namespace mozilla { namespace gfx { class DrawTarget; } }
 
 class THEBES_API gfxPlatformMac : public gfxPlatform {
 public: