Convert resources to take an instance key instead of an Instance*.

ppapi/cpp/dev/font_dev.h

Index: ppapi/cpp/dev/font_dev.h
diff --git a/ppapi/cpp/dev/font_dev.h b/ppapi/cpp/dev/font_dev.h
index a9604e939529688bdff2277c992a598a5aa32e14..d0875a50cc3fe599bb7a388f7a950c8f7d1bd5c8 100644
--- a/ppapi/cpp/dev/font_dev.h
+++ b/ppapi/cpp/dev/font_dev.h
@@ -16,7 +16,7 @@ struct PP_FontDescription_Dev;
 namespace pp {
 
 class ImageData;
-class Instance;
+class InstanceHandle;
 class Point;
 class Rect;
 
@@ -103,13 +103,14 @@ class Font_Dev : public Resource {
   Font_Dev();
 
   explicit Font_Dev(PP_Resource resource);
-  Font_Dev(Instance* instance, const FontDescription_Dev& description);
+  Font_Dev(const InstanceHandle& instance,
+           const FontDescription_Dev& description);
   Font_Dev(const Font_Dev& other);
 
   Font_Dev& operator=(const Font_Dev& other);
 
   // PPB_Font methods:
-  static Var GetFontFamilies(Instance* instance);
+  static Var GetFontFamilies(const InstanceHandle& instance);
   bool Describe(FontDescription_Dev* description,
                 PP_FontMetrics_Dev* metrics) const;
   bool DrawTextAt(ImageData* dest,