Cleanup PreferenceMACs on browser_tests startup

content/public/test/test_launcher.h

Index: content/public/test/test_launcher.h
diff --git a/content/public/test/test_launcher.h b/content/public/test/test_launcher.h
index b58f9b75d78973ff3c38e424d1a1eac5dcca5675..7acd114943157b4acfc26cb8d6474429c1442a7a 100644
--- a/content/public/test/test_launcher.h
+++ b/content/public/test/test_launcher.h
@@ -58,6 +58,10 @@ class TestLauncherDelegate {
       base::CommandLine* command_line,
       base::TestLauncher::LaunchOptions* test_launch_options);
 
+  // Allows a TestLauncherDelegate to do work before the launcher shards test
+  // jobs.
+  virtual void PreSharding() {}
+
   // Allows a TestLauncherDelegate to adjust the number of |default_jobs| used
   // when --test-launcher-jobs isn't specified on the command-line.
   virtual void AdjustDefaultParallelJobs(int* default_jobs) {}