Rework how MB and GN handle concurrent links.

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 0670acd914533c6c4a7ddee5e8ed076871ee0032..24d7bc3d8ec979f092f2775ec39cd4f76d68d3a2 100644
--- a/tools/mb/mb_config.pyl
+++ b/tools/mb/mb_config.pyl
@@ -641,9 +641,9 @@
       'precise64 beta': 'gn_official',
       'precise64 stable': 'gn_official',
       'precise64 trunk': 'gn_official',
-      'win beta': 'gyp_official',
-      'win stable': 'gyp_official',
-      'win trunk': 'gyp_official',
+      'win beta': 'gyp_official_six_concurrent_links',
+      'win stable': 'gyp_official_six_concurrent_links',
+      'win trunk': 'gyp_official_six_concurrent_links',
     },
 
     'tryserver.blink': {
@@ -1341,6 +1341,10 @@
       'gyp', 'official', 'syzyasan',
     ],
 
+    'gyp_official_six_concurrent_links': [
+      'gyp', 'official', 'six_concurrent_links',
+    ],
+
     # TODO(crbug.com/595947) - figure out how to handle PGO, which needs
     # to invoke GYP/GN twice, with two different sets of flags, apparently.
     'gyp_official_winpgo': [
@@ -2242,6 +2246,13 @@
       'mixins': ['shared', 'release', 'goma']
     },
 
+    'six_concurrent_links': {
+      # TODO(crbug.com/611491) Adjust the get_concurrent_links script
+      # to be more conservative so that we don't need this.
+      'gn_args': 'concurrent_links=6',
+      'gyp_defines': 'gyp_link_concurrency=6',
+    },
+
     'static': {
       'gn_args': 'is_component_build=false',
       'gyp_defines': 'component=static_library',