Update APIs to use new Google cloud paths.

grpc/discovery/registry.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package discovery
 
 import (
         "bytes"
         "compress/gzip"
         "io"
         "io/ioutil"
         "sync"
 
         "github.com/golang/protobuf/proto"
 
-        "github.com/luci/luci-go/common/proto/google/descriptor"
+        "google.golang.org/genproto/protobuf"
 )
 
 type entry struct {
         compressedBytes []byte
 
         init        sync.Once
         unmarshaled *descriptor.FileDescriptorSet
         err         error
 }
 
@@ skipping 51 matching lines @@
 
                 var unmarshaled descriptor.FileDescriptorSet
                 e.err = proto.Unmarshal(uncompressed, &unmarshaled)
                 if e.err != nil {
                         return
                 }
                 e.unmarshaled = &unmarshaled
         })
         return e.unmarshaled, e.err
 }