Lazily require types when validating Extensions API calls

      Custom types now have their own JS files, which is referenced
by the 'js_module' key in the schema. This makes it so $ref
types can be referenced by other APIs without having to load
the whole API of the type referenced.

BUG=222156
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/12647017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@189839 0039d316-1c4b-4281-b951-d872f2087c98