Move WebGPUDecoderImpl to separate files

WebGPUDecoderImpl will later need to link against symbols
in libdawn_native. This patch directly copies the contents to
separate files which later can be conditionally compiled when
building with use_dawn.

Bug: 877147
Change-Id: Iab7abfa2dd636bb4d5d3f3b0a416f2c1f69de990
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504495Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638222}

gpu/command_buffer/service/BUILD.gn

@@ -257,6 +257,8 @@ target(link_target_type, "gles2_sources") {
     "webgpu_cmd_validation_implementation_autogen.h",
     "webgpu_decoder.cc",
     "webgpu_decoder.h",
+    "webgpu_decoder_impl.cc",
+    "webgpu_decoder_impl.h",
     "wrapped_sk_image.cc",
     "wrapped_sk_image.h",
   ]

gpu/command_buffer/service/webgpu_decoder_impl.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef GPU_COMMAND_BUFFER_SERVICE_WEBGPU_DECODER_IMPL_H_
+#define GPU_COMMAND_BUFFER_SERVICE_WEBGPU_DECODER_IMPL_H_
+
+#include "gpu/gpu_gles2_export.h"
+
+namespace gpu {
+
+class CommandBufferServiceBase;
+class DecoderClient;
+
+namespace gles2 {
+class Outputter;
+}  // namespace gles2
+
+namespace webgpu {
+
+class WebGPUDecoder;
+
+GPU_GLES2_EXPORT WebGPUDecoder* CreateWebGPUDecoderImpl(
+    DecoderClient* client,
+    CommandBufferServiceBase* command_buffer_service,
+    gles2::Outputter* outputter);
+
+}  // namespace webgpu
+}  // namespace gpu
+
+#endif  // GPU_COMMAND_BUFFER_SERVICE_WEBGPU_DECODER_IMPL_H_