Doc server: support interfaces in src/extensions

This adds primitive support to the doc server for
API models living in src/extensions/common/api.

BUG=349953
NOTRY=true

Review URL: https://codereview.chromium.org/193383003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256259 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256259