Docserver: Fix the extreme slowness that preview.py takes to render some pages.
This gets it down from 20+ seconds to <2 seconds. The main expensive operation
was transitively finding the availability for every API $ref'd from the page
being rendered. The second most expensive operation was rendering samples.

R=jyasskin@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/50643004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232135 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232135