Update APIs to use new Google cloud paths.

examples/appengine/helloworld_standard/proto/pb.discovery.go

 // AUTOGENERATED. DO NOT EDIT.
 
 package helloworld
 
 import discovery "github.com/luci/luci-go/grpc/discovery"
 
-import "github.com/luci/luci-go/common/proto/google/descriptor"
+import "google.golang.org/genproto/protobuf"
 
 func init() {
         discovery.RegisterDescriptorSetCompressed(
                 []string{
                         "helloworld.Greeter",
                 },
                 []byte{31, 139,
                         8, 0, 0, 9, 110, 136, 0, 255, 116, 147, 65, 111, 211, 78,
                         16, 197, 189, 142, 235, 38, 147, 244, 31, 119, 255, 73, 85, 69,
                         72, 140, 114, 160, 28, 192, 105, 131, 224, 130, 64, 170, 170, 10,
@@ skipping 46 matching lines @@
 //
 // Do NOT modify the returned descriptor.
 func FileDescriptorSet() *descriptor.FileDescriptorSet {
         // We just need ONE of the service names to look up the FileDescriptorSet.
         ret, err := discovery.GetDescriptorSet("helloworld.Greeter")
         if err != nil {
                 panic(err)
         }
         return ret
 }