message: Deflake: attempt to deflake ipro_noop
  This may help, but as jmarantz noted in the review this may not be a complete/thorough fix.
  But hopes are it will improve test stability just enonough for now.
  Added a TODO on what can be done to further improve.
diff: @@ -48,9 +48,23 @@ URL1="${URL}&PageSpeedNoop=$RANDOM1"
  URL2="${URL}&PageSpeedNoop=$RANDOM2"
  fetch_until "$URL1" "grep -c image/webp" 1 --save-headers
  #NUM_REWRITES_URL1=$(scrape_stat image_rewrites)
  +echo -n Waiting for quiescence by checking serf_fetch_active_count ...
  +# The last check in this test was observed to flake. Let's see if waiting
  +# for quiescence here stabilizes it.
  +# TODO(oschaaf): It would be good to keep a count of active rewrite drivers as
  +# well for this purpose. Serf is only one of the things that can finish
  +# asynchronously; slow rewrites can as well.
  +while [ $(scrape_stat serf_fetch_active_count) -gt 0 ]; do
  + echo -n .
  + sleep .1
  +done
  NUM_FETCHES_URL1=$(scrape_stat http_fetches)
  check $WGET -q $WGET_ARGS --save-headers "$URL2" -O $WGET_OUTPUT
  #NUM_REWRITES_URL2=$(scrape_stat image_rewrites)
  +while [ $(scrape_stat serf_fetch_active_count) -gt 0 ]; do
  + echo -n .
  + sleep .1
  +done
  NUM_FETCHES_URL2=$(scrape_stat http_fetches)
  check_from "$(extract_headers $WGET_OUTPUT)" grep -q "image/webp"
  #check [ $NUM_REWRITES_URL2 = $NUM_REWRITES_URL1 ]
  