Reset chromium checkout before syncing

ct/go/worker_scripts/run_chromium_perf/main.go

 // run_chromium_perf is an application that runs the specified benchmark over CT's
 // webpage archives.
 package main
 
 import (
         "encoding/csv"
         "flag"
         "fmt"
         "io/ioutil"
         "os"
@@ skipping 47 matching lines @@
         }
         if *runIDWithPatch == "" {
                 glog.Error("Must specify --run_id_withpatch")
                 return
         }
         if *benchmarkName == "" {
                 glog.Error("Must specify --benchmark_name")
                 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)
                 return
         }
 
         // Create the task file so that the master knows this worker is still busy.
         skutil.LogErr(util.CreateTaskFile(util.ACTIVITY_RUNNING_CHROMIUM_PERF))
         defer util.DeleteTaskFile(util.ACTIVITY_RUNNING_CHROMIUM_PERF)
 
@@ skipping 241 matching lines @@
         }
         writer := csv.NewWriter(csvFile)
         defer writer.Flush()
         for _, row := range [][]string{headers, values} {
                 if err := writer.Write(row); err != nil {
                         return fmt.Errorf("Could not write to %s: %s", csvPath, err)
                 }
         }
         return nil
 }