GitLab

Allow user to define pool of limited size to limit the number of
concurrent tasks that are executed for a given set of tools. The
pool object directly correspond to a ninja pool and can be shared
by multiple targets.

Design document:
https://docs.google.com/document/d/1b598i4WmeFOe3_iRBrjPKSXdcR5R32UPtgxqKmoY2-M/view

BUG=612786

Review-Url: https://codereview.chromium.org/2006923004
Cr-Commit-Position: refs/heads/master@{#397917}