Collect code coverage from the available invocation counts.
The granularity is at function level, and invocation counts may
be lost to GC.

Coverage::Collect returns a std::vector of Coverage::ScriptData.
Each ScriptData contains a script ID and a std::vector of
Coverage::RangeEntry.
Each RangeEntry consists of a end position and the invocation
count. The start position is implicit from the end position of
the previous RangeEntry, or 0 if it's the first RangeEntry.

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

Review-Url: https://codereview.chromium.org/2689493002
Cr-Commit-Position: refs/heads/master@{#43072}