Add gpu::MailboxHolder to hold state for a gpu::Mailbox

cc/layers/delegated_frame_provider_unittest.cc

Index: cc/layers/delegated_frame_provider_unittest.cc
diff --git a/cc/layers/delegated_frame_provider_unittest.cc b/cc/layers/delegated_frame_provider_unittest.cc
index 20b355c6192bcbb0bd071343dbbc13b1f061edc9..8a711f043187ef7e5580348c10abdfd75570ce78 100644
--- a/cc/layers/delegated_frame_provider_unittest.cc
+++ b/cc/layers/delegated_frame_provider_unittest.cc
@@ -37,7 +37,7 @@ class DelegatedFrameProviderTest
                                ResourceProvider::ResourceId resource_id) {
     TransferableResource resource;
     resource.id = resource_id;
-    resource.target = GL_TEXTURE_2D;
+    resource.mailbox_holder.texture_target = GL_TEXTURE_2D;
     frame->resource_list.push_back(resource);
   }