Change the default to false. Block coverage will need to be
enabled explicitly via inspector protocol, which is already
being done.

R=franzih@chromium.org

Bug: v8:6738
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I08684ce7b501981bc376a6bc6181fabac9628a63
Reviewed-on: https://chromium-review.googlesource.com/689234Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48235}