TestResultsServer: special handling for aggregate file requests.

If the buildnumber parameter is omitted when requesting an aggregate
file, the file contents should be served up, rather than a file
list.

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/419883002

git-svn-id: svn://svn.chromium.org/blink/trunk@178940 bbb929c8-8fbe-4397-9dbb-9b2b20218538