The isolates step times out due to some very slow tests. Marking as slow
changes the test order and increases throughput.

NOTRY=true
TBR=yangguo@chromium.org

Change-Id: Iaaf6fe93e7f0e17266923d1ab6f0fe7b09abea9e
Reviewed-on: https://chromium-review.googlesource.com/1213212Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55734}