[Content] Attributes 'modifiedDate' and 'releaseDate' defined in 'Conten...

...t' interface should be 'Date' type

Add conversion from 'magic' database format to JS Date for given fields.

BUG=XWALK-2803
Since Date object support constuture with datestring with format "%Y-%m-%d %H:%M:%S", I'd suggest pass the date as that format in native code instead of format "%Y:%m:%d %H:%M:%S". Go through the code, I think `setReleaseDate()` and `setModifiedDate()`  should be good place to do that.Hint: strptime and strftime can be used.

BTW, the C++ code on class `ContentItem` is not followed the [chromium naming rule](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Naming), please rework:
* variable or member names: `releaseDate_` -> `release_date_`
* Accessors: `releaseDate()` -> `release_date() `
* Mutators: `setReleaseDate()` -> `set_release_date()`