Add "detailed" flag to Profiler.startPreciseCoverage to specify
granularity (block coverage vs function coverage).

The default value is currently set to FLAG_block_coverage, which
is currently true. This is so that the V8 roll does not break
LayoutTests. I'll set it to false once I made changes to Blink.

R=jgruber@chromium.org, pfeldman@chromium.org

Bug: v8:6738
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I7242e897ab02713188a5292ca8c8bb58985e3a9b
Reviewed-on: https://chromium-review.googlesource.com/625616Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47533}