This is so that precise coverage starts with a clean slate.
The old behavior can be emulated by calling getBestEffortCoverage
before starting precise coverage.

R=jgruber@chromium.org

Bug: chromium:757998
Change-Id: Ib3ee2316966f676456198159bdcf8ba8b9d3896f
Reviewed-on: https://chromium-review.googlesource.com/635084
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47629}