Docserver: Make FileSystem.ReadSingle return a Future rather than immediately
resolving the fetch from FileSystem.Read. As I chip away at what is making cron
runs take so long, all expensive operations should return Futures.

While this patch does add syntactic noise, it's for the best - it should help
identity places where resolving the Futures immediately is unnecessary, and
those can be fixed incrementally.

BUG=305280
R=jyasskin@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/26629002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227868 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227868