Add custom field converter to JSONValueConverter.

In the real JSON results, sometimes string value does not literally
mean a string, but labels (enum), URLs, or timestamps.  Custom
field accepts those string values and converting functions for
such fields.

See http://codereview.chromium.org/9147060 for the real use cases.

BUG=109375
TEST=passed

Review URL: http://codereview.chromium.org/9184002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@117580 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed