If the top-level function cannot be found, we previously assumed an
invocation count of 1. This is wrong when we expect the invocation
counts to be reset for precise coverage.

TBR=jgruber@chromium.org
R=caseq@chromium.org,pfeldman@chromium.org
BUG=v8:5808

Review-Url: https://codereview.chromium.org/2723003007
Cr-Commit-Position: refs/heads/master@{#43620}