Support explicit pools in actions

This change allows explicitly specifying pools for actions.
Furthermore, it is also possible to specify a default pool for
actions as part of the toolchain. Pools can be also defined as
console ones to emulate the original console attribute behavior.

BUG=635308

Review-Url: https://codereview.chromium.org/2926013002
Cr-Commit-Position: refs/heads/master@{#483252}
19 files changed