goprotobuf: Changes for new struct tag format.

Protocol messages have changed their struct tags from
  "PB(...)"
to
  `protobuf:"..."`
The format represented by "..." is unchanged.

Regenerate {descriptor,plugin}.pb.go files at the same time.

R=r, rsc
CC=golang-dev
http://codereview.appspot.com/4654077
diff --git a/proto/extensions.go b/proto/extensions.go
index 4594c8b..f44d741 100644
--- a/proto/extensions.go
+++ b/proto/extensions.go
@@ -62,7 +62,7 @@
 	ExtensionType interface{} // nil pointer to the extension type
 	Field         int32       // field number
 	Name          string      // fully-qualified name of extension
-	Tag           string      // PB(...) tag style
+	Tag           string      // protobuf tag style
 }
 
 // isExtensionField returns true iff the given field number is in an extension range.