Docserver: Support markdown for HTML content.

R=kalman@chromium.org
BUG=293681

Review URL: https://codereview.chromium.org/93743005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243980 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243980