ABANDONED: Update base::JSONValueConverter to support custom actions.

Update base::JSONValueConverter to support custom actions.

The existing RegisterCustomField() method requires that the
caller provide a conversion function and specify the
location where the resulting value should be saved.

This change adds RegisterCustomAction() and
RegisterRepeatedCustomAction(), which can be used to add
handlers that perform arbitrary actions.  For example, this
is useful if you have JSON like this:

  {"link": [
    {"type": "email",
     "href": "me@example.org"},
    {"type": "home",
     "href": "http://www.example.org"}
  ]}

and you want to store it in a class like:

  struct Links {
    std::string email_href;
    std::string home_href;
  };

RegisterRepeatedCustomAction() can be used to register a
function that that receives a DictionaryValue for to each
element in the "link" list and saves its "href" field to the
appropriate member in Links depending on the value of the
"type" field.

BUG=128805
TEST=added

[Daniel Erat, 2012-05-30 23:18:14]

https://chromiumcodereview.appspot.com/10440101/diff/1/base/json/json_value_c...
File base/json/json_value_converter.h (right):

https://chromiumcodereview.appspot.com/10440101/diff/1/base/json/json_value_c...
base/json/json_value_converter.h:105: class FieldHandlerBase {
I renamed this since it's not just used for conversion now.

[Jun Mukai, 2012-05-30 23:51:10]

lgtm

https://chromiumcodereview.appspot.com/10440101/diff/2001/base/json/json_valu...
File base/json/json_value_converter_unittest.cc (right):

https://chromiumcodereview.appspot.com/10440101/diff/2001/base/json/json_valu...
base/json/json_value_converter_unittest.cc:34: std::string home_href;
Can you define another struct for custom actions?
I feel the "SimpleMessage" got a bit complicated now.