Add error messages to JSONReader and friends. This required a bit of refactor...

chrome/common/json_value_serializer.h

Index: chrome/common/json_value_serializer.h
===================================================================
--- chrome/common/json_value_serializer.h	(revision 6390)
+++ chrome/common/json_value_serializer.h	(working copy)
@@ -41,8 +41,10 @@
   // in to the constructor into a structure of Value objects.  If the return
   // value is true, the |root| parameter will be set to point to a new Value
   // object that corresponds to the values represented in the string.  The
-  // caller takes ownership of the returned Value objects.
-  bool Deserialize(Value** root);
+  // caller takes ownership of the returned Value objects. Otherwise, the root
+  // value will be changed and if |error_message| is non-null, it will contain
+  // a string describing the error.
+  bool Deserialize(Value** root, std::string* error_message);
 
   void set_pretty_print(bool new_value) { pretty_print_ = new_value; }
   bool pretty_print() { return pretty_print_; }
@@ -86,8 +88,10 @@
   // in to the constructor into a structure of Value objects.  If the return
   // value is true, the |root| parameter will be set to point to a new Value
   // object that corresponds to the values represented in the file.  The
-  // caller takes ownership of the returned Value objects.
-  bool Deserialize(Value** root);
+  // caller takes ownership of the returned Value objects. Otherwise, the root
+  // value will be changed and if |error_message| is non-null, it will contain
+  // a string describing the error.
+  bool Deserialize(Value** root, std::string* error_message);
 
  private:
   std::wstring json_file_path_;