Add a way to compute the code coverage of fuzzers

This also fixes bin/coverage.

fuzz/coverage borrows heavily from bin/coverage.
BUG=skia:

Change-Id: I9e353d1f5ea3bca1d57d66b1c1ecabc6f9b23cee
Reviewed-on: https://skia-review.googlesource.com/8414
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
3 files changed