Since css/css-color/color-resolving-hsl.html runs over 1000 tests, it
can sometimes run past the timeout. Mark it as slow and reduce the
amount of calculations the test has to perform.

Bug:
Change-Id: If494b36924733dc67c0d40f572df28a30d4006c4
Reviewed-on: https://chromium-review.googlesource.com/837705
Reviewed-by: Eric Willigers <[email protected]>
Commit-Queue: Eric Willigers <[email protected]>
Cr-Commit-Position: refs/heads/master@{#525550}