infra: Increase Linux Goma RBE LoadTests to -j300

This will increase the load. We can adjust the overall load by turning
individual builders on/off.

R=tikuta@chromium.org, ukai@chromium.org, yyanagisawa@chromium.org

Bug: 945731
Change-Id: I30792178e164acc210785d3a7670e0b0159de996
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539489
Commit-Queue: Simon Que <sque@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644685}

infra/config/cr-buildbucket.cfg

@@ -2638,24 +2638,28 @@ buckets {
       dimensions: "os:Ubuntu-14.04"
       mixins: "experimental"
       mixins: "goma-ci"
+      mixins: "goma-j300"
     }
     builders {
       name: "Chromium Linux Goma RBE LoadTest (debug)"
       dimensions: "os:Ubuntu-14.04"
       mixins: "experimental"
       mixins: "goma-ci"
+      mixins: "goma-j300"
     }
     builders {
       name: "Chromium Linux Goma RBE LoadTest (clobber)"
       dimensions: "os:Ubuntu-14.04"
       mixins: "experimental"
       mixins: "goma-ci"
+      mixins: "goma-j300"
     }
     builders {
       name: "Chromium Linux Goma RBE LoadTest (clobber) (debug)"
       dimensions: "os:Ubuntu-14.04"
       mixins: "experimental"
       mixins: "goma-ci"
+      mixins: "goma-j300"
     }
     builders {
       name: "Chromium Linux Goma RBE LoadTest (ATS)"