Collecting precise invocation counts need to be explicitly
enabled. Once enabled, we disable optimization (optimized
code does not increment invocation count, and may inline
callees), and make sure feedback vectors interesting for
code coverage is not garbage-collected.

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

Review-Url: https://codereview.chromium.org/2686063002
Cr-Commit-Position: refs/heads/master@{#43082}