Before adding another test for removing breakpoint, this CL modernizes
the existing test for setting breakpoints.

R=kozy@chromium.org
CC=ahaas@chromium.org

Bug: chromium:837572
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I642f9673f327f4ec569a4f67a61b5e264cf25b8f
Reviewed-on: https://chromium-review.googlesource.com/c/1264636Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56514}