If Coverage goes out of scope, ScriptData, FunctionData, or BlockData still rely on 
Coverage's coverage_. Make coverage_ a shared_ptr owned by all four classes. 

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ifab5d05184cc5db0fd0a935254b967286295e63f
Reviewed-on: https://chromium-review.googlesource.com/657381Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47938}