Add a warning when developing an extension that uses old manifest version.

chrome/common/extensions/manifest.h

Index: chrome/common/extensions/manifest.h
diff --git a/chrome/common/extensions/manifest.h b/chrome/common/extensions/manifest.h
index 3e82f38eb7302c0fd25ab82cf4bb821525b47092..9c3f55797bb50d5bab69879447d052969f96eae7 100644
--- a/chrome/common/extensions/manifest.h
+++ b/chrome/common/extensions/manifest.h
@@ -33,7 +33,7 @@ class Manifest {
   // be populated if there are keys in the manifest that cannot be specified by
   // the extension type.
   void ValidateManifest(std::string* error,
-                        std::vector<std::string>* warnings) const;
+                        Extension::InstallWarningVector* warnings) const;
 
   // The version of this extension's manifest. We increase the manifest
   // version when making breaking changes to the extension system. If the