Prevent parallelization of ash browsertests on the same machine.

scripts/master/factory/chromium_factory.py

Index: scripts/master/factory/chromium_factory.py
diff --git a/scripts/master/factory/chromium_factory.py b/scripts/master/factory/chromium_factory.py
index cff6c1c9f7b40f197d485e816fe15b8d5c637524..4914dfa6c37c5d530381e146c9aa97dc7a53fe15 100644
--- a/scripts/master/factory/chromium_factory.py
+++ b/scripts/master/factory/chromium_factory.py
@@ -537,7 +537,10 @@ class ChromiumFactory(gclient_factory.GClientFactory):
       f.AddBuildrunnerGTest('content_browsertests', fp)
     if R('ash_browsertests'):
       ash_fp = fp.copy()
-      ash_fp['browser_tests_extra_options'] = ['--ash-browsertests']
+      # Ash browser tests use the Metro environment and thus cannot be ran in
+      # parallel on the same machine.
+      ash_fp['browser_tests_extra_options'] = ['--ash-browsertests',
+                                               '--test-launcher-jobs=1']
       f.AddBuildrunnerBrowserTests(ash_fp)
 
     # Big, UI tests: