Add a per-thread proc table using thread local storage

In situations where both dawn_wire and dawn_native are used on separate
threads (Chrome with --single-process or --in-process-gpu), it's
desirable to have a per-thread proc table so that the WebGPU C++ API can
still be used. This eliminates classes of bugs with manual
reference/release errors.

This also changes many of the GetProcs functions to return const
references to the static proc tables known at compile time, instead of a
copy.

Bug: none
Change-Id: I8775bb715b312dd9476a1903fbd797d4b1302614
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/29240
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/include/dawn/dawn_thread_dispatch_proc.h

@@ -0,0 +1,33 @@
+// 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.
+
+#ifndef DAWN_DAWN_THREAD_DISPATCH_PROC_H_
+#define DAWN_DAWN_THREAD_DISPATCH_PROC_H_
+
+#include "dawn/dawn_proc.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Call dawnProcSetProcs(&dawnThreadDispatchProcTable) and then use dawnProcSetPerThreadProcs
+// to set per-thread procs.
+WGPU_EXPORT extern DawnProcTable dawnThreadDispatchProcTable;
+WGPU_EXPORT void dawnProcSetPerThreadProcs(const DawnProcTable* procs);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // DAWN_DAWN_THREAD_DISPATCH_PROC_H_