Use swarming in capture_archives CT task and capture archives in parallel

ct/go/worker_scripts/worker_common/worker_common.go

 /*
         Common initialization for worker scripts.
 */
 
 package worker_common
 
 import (
         "flag"
+        "os"
+
+        "github.com/skia-dev/glog"
 
         "go.skia.org/infra/ct/go/util"
         "go.skia.org/infra/go/common"
+        "go.skia.org/infra/go/exec"
+        skutil "go.skia.org/infra/go/util"
 )
 
 var (
         Local = flag.Bool("local", false, "Running locally if true. As opposed to in production.")
 )
 
 func Init() {
         common.Init()
         if *Local {
                 util.SetVarsForLocal()
+        } else {
+                // Add depot_tools to the PATH.
+                skutil.LogErr(os.Setenv("PATH", os.Getenv("PATH")+":"+util.DepotToolsDir))
+                // Bring up Xvfb on workers (for GCE instances).
+                if _, _, err := exec.RunIndefinitely(&exec.Command{
+                        Name:        "sudo",
+                        Args:        []string{"Xvfb", ":0", "-screen", "0", "1280x1024x24"},
+                        Env:         []string{},
+                        InheritPath: true,
+                        Timeout:     util.XVFB_TIMEOUT,
+                        LogStdout:   true,
+                        Stdout:      nil,
+                        LogStderr:   true,
+                        Stderr:      nil,
+                }); err != nil {
+                        // CT's baremetal machines will already have an active display 0.
+                        glog.Infof("Could not run Xvfb on Display 0: %s", err)
+                }
         }
 }