Enables block coverage by default.

Design doc: http://goo.gl/hSJhXn
Tracking bug: http://crbug.com/v8/6000

Bug: v8:6000
Change-Id: I8c56474473b60e4707b75dc601b3e88455861a27
Reviewed-on: https://chromium-review.googlesource.com/583093
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46898}