rebaseline_server: move more code into common path for GET_live_results and GET_prefetch_results

patchset 1 from issue 467843002

BUG=skia:1918
NOTREECHECKS=true
NOTRY=true
R=stephana@google.com
TBR=stephana

Author: epoger@google.com

Review URL: https://codereview.chromium.org/465143004
1 file changed