[gn] Hard-code msvs version 2013 in MB.

tools/mb/mb.py

Index: tools/mb/mb.py
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 536dc00fcb16c4148a9f7a0384b55c1b8fc0a850..6c7c7735ebbe9a3fb2026cf61e148914259a9528 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -36,6 +36,13 @@ sys.path = [os.path.join(CHROMIUM_SRC_DIR, 'build')] + sys.path
 import gn_helpers
 
 
+# We override this here as some legacy gyp scripts are still called through gn.
+# They rely on the environment variable or otherwise use chromium's default of
+# 2015.
+# TODO(machenbach): For switching to msvs 2015, simply remove this.
+MSVS_VERSION = '2013'
+
+
 def main(args):
   mbw = MetaBuildWrapper()
   return mbw.Main(args)
@@ -777,7 +784,12 @@ class MetaBuildWrapper(object):
       self.WriteFile(gn_runtime_deps_path, '\n'.join(gn_labels) + '\n')
       cmd.append('--runtime-deps-list-file=%s' % gn_runtime_deps_path)
 
-    ret, _, _ = self.Run(cmd)
+    # Override msvs infra environment variables.
+    env = {}
+    env.update(os.environ)
+    env['GYP_MSVS_VERSION'] = MSVS_VERSION
+
+    ret, _, _ = self.Run(cmd, env=env)
     if ret:
         # If `gn gen` failed, we should exit early rather than trying to
         # generate isolates. Run() will have already logged any error output.