Revert "gn: Remove Scheduler::pool()."

Revert "gn: Remove Scheduler::pool()."

This reverts commit 5476ec6bf7ccbada1734a0cdec7d570bb042aa30.

Reason: that change causes GN to occasionally fail to load any
input files, especially when there is load on the build machine.

Errors look like:
  ERROR No targets.
  I could not find any targets to write, so I'm doing nothing.
usually preceeded by a stray "Build argument has no effect" warning.

Original issue: https://codereview.chromium.org/2515383005

BUG=674213
R=brettw@chromium.org,dpranke@chromium.org

[fdoray, 2017-01-06 22:31:39]

How can we repro this and verify that the revert fixes the problem?

[Dirk Pranke, 2017-01-06 22:36:43]

On 2017/01/06 22:31:39, fdoray wrote:
> How can we repro this and verify that the revert fixes the problem?

Comments #13 and #14 on the bug give some pointers. 

I think this is a plausible diagnosis and fix, but I want brettw@ to review it
and be sure.

[brettw, 2017-01-06 23:45:04]

This is awesome! After digging into why this fixes the issue I will post a patch
I think fixes it better.

[brettw, 2017-01-06 23:55:34]

Alternative fix in https://codereview.chromium.org/2617253002

[tsniatowski, 2017-01-07 09:05:02]

Description was changed from

==========
Revert "gn: Remove Scheduler::pool()."

This reverts commit 5476ec6bf7ccbada1734a0cdec7d570bb042aa30.

Reason: that change causes GN to occasionally fail to load any
input files, especially when there is load on the build machine.

Errors look like:
  ERROR No targets.
  I could not find any targets to write, so I'm doing nothing.
usually preceeded by a stray "Build argument has no effect" warning.

BUG=674213
R=brettw@chromium.org,dpranke@chromium.org
==========

to

==========
Revert "gn: Remove Scheduler::pool()."

This reverts commit 5476ec6bf7ccbada1734a0cdec7d570bb042aa30.

Reason: that change causes GN to occasionally fail to load any
input files, especially when there is load on the build machine.

Errors look like:
  ERROR No targets.
  I could not find any targets to write, so I'm doing nothing.
usually preceeded by a stray "Build argument has no effect" warning.

Original issue: https://codereview.chromium.org/2515383005

BUG=674213
R=brettw@chromium.org,dpranke@chromium.org
==========

[tsniatowski, 2017-01-10 08:49:52]

On 2017/01/06 23:55:34, brettw (ping after 24h) wrote:
> Alternative fix in https://codereview.chromium.org/2617253002

Dropping this because the issues were fixed on master without needing a revert.