The default TextContentHandler::makeEmptyContent() implementation
is an empty string, which is not valid JSON.
Change-Id: I8bc6ec647c5e8cd01c5cd63031525619c4ef44d0
* All content handlers that deal with code/data tend to have
English as their page language & pageview language, so moved common
code to the abstract CodeContentHandler class.
* Renamed JSONContent & JSONContentHandler into JsonContent*
Change-Id: I46819a0572ef5becc211d0d82471ff7102edaa3c