Added SyncToken command buffer trait to help with IPC messages.

gpu/command_buffer/service/mailbox_manager.h

Index: gpu/command_buffer/service/mailbox_manager.h
diff --git a/gpu/command_buffer/service/mailbox_manager.h b/gpu/command_buffer/service/mailbox_manager.h
index 78fb8f882e03bf4166bc8fca1cc52b41b856b937..a914f5e83713d4dbda3d7afa0afa6ffadb299a34 100644
--- a/gpu/command_buffer/service/mailbox_manager.h
+++ b/gpu/command_buffer/service/mailbox_manager.h
@@ -10,10 +10,12 @@
 #include "gpu/gpu_export.h"
 
 namespace gpu {
+
+struct SyncToken;
+
 namespace gles2 {
 
 class Texture;
-struct SyncToken;
 
 // Manages resources scoped beyond the context or context group level.
 class GPU_EXPORT MailboxManager : public base::RefCounted<MailboxManager> {