Just remembered, the same thing already happened with the other special layout test try bot (spv2 try bot,  bug 703478 ).

Another possible solution that wouldn't necessarily involve adding more slaves would be to re-use slaves by making one pool of four slaves that's used by multiple builders; for example we could join this builder with the spv2 try bot, and have this config:

  for slave in (775, 1404, 1403, 1405):
    result.append(
      {
        'master': 'TryServerChromiumLinux',
        'os': 'linux',
        'version': 'trusty',
        'bits': '64',
        'builder': [
            'linux_layout_tests_slimming_paint_v2',
            'linux_layout_tests_layout_ng',
        ],
        'hostname': 'slave%d-c4' % slave,
      })

in https://cs.chromium.org/chromium/build/masters/master.tryserver.chromium.linux/slaves.cfg?l=183

pdr@, do you think this would be acceptable if the spv2 layout test try bot was shared? If capacity might still be a problem we could still add more slaves...

Sounds good to me. Lets just keep an eye on the respective build times.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/6fbefca00bc22d0e950f92a0e0cb945d16e6ebf4

commit 6fbefca00bc22d0e950f92a0e0cb945d16e6ebf4
Author: Quinten Yearsley <qyearsley@google.com>
Date: Wed Jun 14 17:44:36 2017

Merge the slave pools of the layout_ng and spv2 try bots.

Bug:  732964 
Change-Id: I190b08a5e12f9d1773afdd6a0e74a07da9a29f55
Reviewed-on: https://chromium-review.googlesource.com/534675
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

[modify] https://crrev.com/6fbefca00bc22d0e950f92a0e0cb945d16e6ebf4/masters/master.tryserver.chromium.linux/slaves.cfg

Master is being restarted; after that, we expect to see https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_layout_tests_layout_ng with multiple slaves listed.

Yes, I can see 4 slaves, and running multiple jobs in parallel. Superb, thank you so much!