Docserver: use webapp2 not webapp.

R=yoz@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/68933004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234282 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234282