Initial work to remove the notion of all extension message payloads as strings.

chrome/renderer/resources/extensions/schema_utils.js

Index: chrome/renderer/resources/extensions/schema_utils.js
diff --git a/chrome/renderer/resources/extensions/schema_utils.js b/chrome/renderer/resources/extensions/schema_utils.js
index a2587a2c95725ee53bf301bcf89df721e5e17401..8cd021287ccce2072084100886e26d96249f9035 100644
--- a/chrome/renderer/resources/extensions/schema_utils.js
+++ b/chrome/renderer/resources/extensions/schema_utils.js
@@ -36,7 +36,8 @@ if (chromeHidden.JSONSchemaValidator) {
         message += ".";
         throw new Error(message);
       } else if (!parameterSchemas[i].optional) {
-        throw new Error("Parameter " + (i + 1) + " is required.");
+        throw new Error("Parameter " + (i + 1) + " (" +
+            parameterSchemas[i].name + ") is required.");
       }
     }
   };