Debug::ClearCoverageInfo may be called multiple times, for example
whenever coverage mode is switched to BestEffort. When encountering a
DebugInfo that does not have a CoverageInfo attached, simply do nothing
instead of failing a DCHECK.

TBR=yangguo@chromium.org

Bug: v8:6000
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I3910b21ab40e7db02af5f3b6add3ec3f86e5bd91
Reviewed-on: https://chromium-review.googlesource.com/588788
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46925}