IPC to generate mailbox names on the GPU process IO thread.

gpu/command_buffer/service/mailbox_manager.h

Index: gpu/command_buffer/service/mailbox_manager.h
===================================================================
--- gpu/command_buffer/service/mailbox_manager.h	(revision 166046)
+++ gpu/command_buffer/service/mailbox_manager.h	(working copy)
@@ -56,6 +56,10 @@
   // manager.
   void DestroyOwnedTextures(TextureManager* owner, bool have_context);
 
+  std::string private_key() {
+    return std::string(private_key_, sizeof(private_key_));
+  }
+
  private:
   friend class base::RefCounted<MailboxManager>;
 
@@ -86,6 +90,7 @@
       std::pointer_to_binary_function<
           const TargetName&, const TargetName&, bool> > TextureDefinitionMap;
 
+  char private_key_[GL_MAILBOX_SIZE_CHROMIUM / 2];
   crypto::HMAC hmac_;
   TextureDefinitionMap textures_;