Update APIs to use new Google cloud paths.

server/static/rpcexplorer/test/gen.go

 package main
 
 // This file generates descriptor.html. Run itself:
 //go:generate go run gen.go
 
 import (
         "fmt"
         "io/ioutil"
         "os"
         "os/exec"
 
         "github.com/golang/protobuf/jsonpb"
         "github.com/golang/protobuf/proto"
-        "github.com/luci/luci-go/common/proto/google/descriptor"
+        "google.golang.org/genproto/protobuf"
 )
 
 func run() error {
         descFile, err := ioutil.TempFile("", "desc")
         if err != nil {
                 return err
         }
 
         protoc := exec.Command(
                 "protoc",
@@ skipping 46 matching lines @@
         _, err = fmt.Fprintln(out, ";\n</script>")
         return err
 }
 
 func main() {
         if err := run(); err != nil {
                 fmt.Fprintln(os.Stderr, err)
                 os.Exit(1)
         }
 }