Docserver: Add a MockFunction class for testing.

R=yoz@chromium.org

Review URL: https://codereview.chromium.org/52713014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232483 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232483