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

ppapi/cpp/file_system.h

Index: ppapi/cpp/file_system.h
diff --git a/ppapi/cpp/file_system.h b/ppapi/cpp/file_system.h
index 73b4813a0c3300d54aa4581b8ad6873d8b70be49..8aea7a555c5fb3ae6a1981df7293738211de609a 100644
--- a/ppapi/cpp/file_system.h
+++ b/ppapi/cpp/file_system.h
@@ -30,11 +30,12 @@ class FileSystem : public Resource {
 
   /// This constructor creates a file system object of the given type.
   ///
-  /// @param[in] instance A <code>Instance</code> identifying the instance
-  /// with the file.
+  /// @param[in] instance The instance with which this resource will be
+  /// associated.
+  ///
   /// @param[in] type A file system type as defined by
   /// <code>PP_FileSystemType</code> enum.
-  FileSystem(Instance* instance, PP_FileSystemType type);
+  FileSystem(const InstanceHandle& instance, PP_FileSystemType type);
 
   /// Open() opens the file system. A file system must be opened before running
   /// any other operation on it.