Add Jinja2 dependency to .gyp in Blink bindings and core

Scripts that use Jinja2 incorrectly fail to depend on it, so not rerun
when library is updated.
This correctly adds the dependency.

Issue identified by Nico in this comment:
https://codereview.chromium.org/23460035/#msg12

Review URL: https://chromiumcodereview.appspot.com/23710043

git-svn-id: svn://svn.chromium.org/blink/trunk@157746 bbb929c8-8fbe-4397-9dbb-9b2b20218538