Local testing suggests that optimized builds add more speed
without trading off tool usability. We get the following
differences (A: non-optimized build, B: optimized):

Sometimes: Lines instrumented in A (covered and uncovered) are not instrumented in B.
Rarely: Lines instrumented and covered in A are instrumented, but not covered in B.

The latter might simply be caused by timing differences in
the two builds.

BUG=chromium:568949
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1719923002

Cr-Commit-Position: refs/heads/master@{#34193}