Rework how MB and GN handle concurrent links.

build/toolchain/concurrent_links.gni

Index: build/toolchain/concurrent_links.gni
diff --git a/build/toolchain/concurrent_links.gni b/build/toolchain/concurrent_links.gni
new file mode 100644
index 0000000000000000000000000000000000000000..9705255b4d48a30725b489bada9666c6caa8a8b5
--- /dev/null
+++ b/build/toolchain/concurrent_links.gni
@@ -0,0 +1,31 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This file should only be imported from files that define toolchains.
+# There's no way to enforce this exactly, but all toolchains are processed
+# in the context of the default_toolchain, so we can at least check for that.
+assert(current_toolchain == default_toolchain)
+
+import("//build/config/sanitizers/sanitizers.gni")
+import("//build/toolchain/toolchain.gni")
+
+declare_args() {
+  # Limit the number of concurrent links; we often want to run fewer
+  # links at once than we do compiles, because linking is memory-intensive.
+  # The default to use varies by platform and by the amount of memory
+  # available, so we call out to a script to get the right value.
+  concurrent_links = -1
+}
+
+if (concurrent_links == -1) {
+  if (allow_posix_link_time_opt || is_cfi) {
+    _args = [ "--lto" ]
+  } else {
+    _args = []
+  }
+
+  # TODO(crbug.com/617429) Pass more build configuration info to the script
+  # so that we can compute better values.
+  concurrent_links = exec_script("get_concurrent_links.py", _args, "value")
+}