Slab-allocate OpenGL bind groups

Now that all backends use slab-allocated bind groups, this patch also
moves the BindGroup implementation with owned-data into the Null backend.

Bug: dawn:340
Change-Id: I08a952075b382008fb82f1fbab3f779cc05bc2a3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16747
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/opengl/BindGroupGL.cpp

@@ -0,0 +1,35 @@
+// Copyright 2020 The Dawn Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "dawn_native/opengl/BindGroupGL.h"
+
+#include "dawn_native/opengl/BindGroupLayoutGL.h"
+#include "dawn_native/opengl/DeviceGL.h"
+
+namespace dawn_native { namespace opengl {
+
+    BindGroup::BindGroup(Device* device, const BindGroupDescriptor* descriptor)
+        : BindGroupBase(this, device, descriptor) {
+    }
+
+    BindGroup::~BindGroup() {
+        ToBackend(GetLayout())->DeallocateBindGroup(this);
+    }
+
+    // static
+    BindGroup* BindGroup::Create(Device* device, const BindGroupDescriptor* descriptor) {
+        return ToBackend(descriptor->layout)->AllocateBindGroup(device, descriptor);
+    }
+
+}}  // namespace dawn_native::opengl