Reset chromium checkout before syncing

ct/go/worker_scripts/run_chromium_perf/main.go

Index: ct/go/worker_scripts/run_chromium_perf/main.go
diff --git a/ct/go/worker_scripts/run_chromium_perf/main.go b/ct/go/worker_scripts/run_chromium_perf/main.go
index 63fdd23d768eec92ecbb2ed2e4c1dbe52af7140f..3be701bd4d1f3eb0af093f11ea812ea74c8b952d 100644
--- a/ct/go/worker_scripts/run_chromium_perf/main.go
+++ b/ct/go/worker_scripts/run_chromium_perf/main.go
@@ -65,6 +65,11 @@ func main() {
 		return
 	}
 
+	// Reset the local chromium checkout.
+	if err := util.ResetCheckout(util.ChromiumSrcDir); err != nil {
+		glog.Errorf("Could not reset %s: %s", util.ChromiumSrcDir, err)
+		return
+	}
 	// Sync the local chromium checkout.
 	if err := util.SyncDir(util.ChromiumSrcDir); err != nil {
 		glog.Errorf("Could not gclient sync %s: %s", util.ChromiumSrcDir, err)