Change how MB handles goma_dir.

tools/mb/mb_config.pyl

Index: tools/mb/mb_config.pyl
diff --git a/tools/mb/mb_config.pyl b/tools/mb/mb_config.pyl
index e2cfe335e1f0b175aaf08638cc2e542e5bf8ef1e..9e1a41626639c2c0026bf7693dca1e9cfaa4a243 100644
--- a/tools/mb/mb_config.pyl
+++ b/tools/mb/mb_config.pyl
@@ -1688,8 +1688,8 @@
     'goma': {
       # The MB code will properly escape goma_dir if necessary in the GYP
       # code path; the GN code path needs no escaping.
-      'gn_args': 'use_goma=true goma_dir="$(goma_dir)"',
-      'gyp_defines': 'use_goma=1 gomadir=$(goma_dir)',
+      'gn_args': 'use_goma=true',
+      'gyp_defines': 'use_goma=1',
     },
 
     'gpu_tests': {