Make extensions/common/api exist.

This is to appease the doc server temporarily until APIs move here.

BUG=None
TBR=kalman@chromium.org

Review URL: https://codereview.chromium.org/194793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256280 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256280