Usually better to have single vm for each builder, but this revealed cache related issue, so now I have an opinion to keep this as-is XP

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/b5d2a9b1b99b02b149b50ad461458bb51ffb65ea

commit b5d2a9b1b99b02b149b50ad461458bb51ffb65ea
Author: Shinya Kawanaka <shinyak@google.com>
Date: Wed Oct 18 10:50:48 2017

Use single vm for each builder for goma staging

I set 2 slaves for 2 builders before, but we'd like to use 1 slave per 1
builder.

Bug:  775807 
Change-Id: I61fb40c314dedd8d12b92f0edd3c6949322147fb
Reviewed-on: https://chromium-review.googlesource.com/725204
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Shinya Kawanaka <shinyak@chromium.org>

[modify] https://crrev.com/b5d2a9b1b99b02b149b50ad461458bb51ffb65ea/masters/master.chromium.goma/builders.pyl