In TestResultsServer, add build_number field to stored files.

R=ojan@chromium.org,dpranke@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/413883003

git-svn-id: svn://svn.chromium.org/blink/trunk@178867 bbb929c8-8fbe-4397-9dbb-9b2b20218538