Use swarming in capture_archives CT task and capture archives in parallel

ct/go/worker_scripts/worker_common/worker_common.go

Index: ct/go/worker_scripts/worker_common/worker_common.go
diff --git a/ct/go/worker_scripts/worker_common/worker_common.go b/ct/go/worker_scripts/worker_common/worker_common.go
index fca0363f0ea4ec08d5b8b13c96e032e227cb3847..de6dd40ec0caf71ea6c6d10a238b58658e0ef6ca 100644
--- a/ct/go/worker_scripts/worker_common/worker_common.go
+++ b/ct/go/worker_scripts/worker_common/worker_common.go
@@ -6,9 +6,14 @@ 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 (
@@ -19,5 +24,23 @@ 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)
+		}
 	}
 }