The inspector uses V8's API handles and should not access
V8 internals. This change makes sure it can use the coverage
data in an encapsulated way.

R=jgruber@chromium.org, kozyatinskiy@chromium.org
BUG=v8:5808

Review-Url: https://codereview.chromium.org/2696163002
Cr-Commit-Position: refs/heads/master@{#43231}