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 5b50656de80b0d6752f64f0d87cc2991701e6997..9965137e3a2a973fe6427e2f019a5c203bba772b 100644
--- a/tools/mb/mb_config.pyl
+++ b/tools/mb/mb_config.pyl
@@ -630,9 +630,9 @@
       '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': {
@@ -1323,6 +1323,10 @@
       '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': [
@@ -2212,6 +2216,13 @@
       '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.

[Dirk Pranke, 2016/06/03 05:44:43]

@scottmg / @brucedawson - thoughts on what we should do w/ the windows link
defaults?

[brucedawson, 2016/06/03 19:42:17]

How much memory do the official builders have? The worst-case six links for
official builds would be six of the 64-bit test binaries which have a combined
linker working set of 82 GB. For smaller binaries the memory footprint can be an
order of magnitude less. Dynamic adjustment would be great but that is
definitely a separate bug (crbug.com/610459).

LTCG linking is multi-threaded in some stages which means that crazy parallelism
is less important on official builds when trying to maximize the hardware.

[Dirk Pranke, 2016/06/05 01:19:49]

looks like 256GB on linux, 128 on win, and 64 on mac.

+      'gn_args': 'concurrent_links=6',
+      'gyp_defines': 'gyp_link_concurrency=6',
+    },
+
     'static': {
       'gn_args': 'is_component_build=false',
       'gyp_defines': 'component=static_library',