Extract the logic to find out the best candidate out of the core of the
scheduler. It allows more flexibility and make it easy to change the
policy use to schedule the basic blocks.

This patch also provide a new algorithm to randomly schedule the code
in order to perform stress tests on the scheduler.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1714753004

Cr-Commit-Position: refs/heads/master@{#34156}