Increase build parallelism for findit builder

The builder currently uses -j50, but it sometimes takes time on compile steps.
https://ci.chromium.org/p/chromium/builders/luci.chromium.findit/findit_variable

This CL increases -j from 50 to 150 for faster culprit CL detection by reducing compile step time.

Change-Id: I035035e14d7a7192434a433f4d3c874b21c25def
Reviewed-on: https://chromium-review.googlesource.com/954647
Commit-Queue: Shuotao Gao <stgao@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542171}

infra/config/global/cr-buildbucket.cfg

@@ -275,6 +275,7 @@ builder_mixins {
 
 builder_mixins {
   name: "findit"
+  mixins: "goma-j150"
   # This category is meant for Findit to trigger tryjobs on.
   category: "Chromium Variable"
   recipe {