Split the libdawn target in components with a single purpose.

The functionality of the dawn_headers and libdawn targets are split into
the following targets:

 - dawn_headers: the new version only exposes the "dawn.h" C API and no
   longer includes the C++ API.
 - dawncpp: the header and implementation of the C++ API that wraps the
   C API. This is unbundled from the rest so the C++ API can be used
   with libdawn_proc or other libraries implementing the C API.
 - libdawn_proc: A DawnProcTable-backend implementation of the C API.

This is needed because in follow-up commit there will be three libraries
implementing the C API: libdawn_proc that trampolines where we want, and
libdawn_native/wire that don't have trampolines for better perf.

BUG=dawn:22

Change-Id: I5d941f0d98e5a4b633e14d67eb5269f7924f0647
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12160
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/include/dawn/dawn_proc.h

@@ -0,0 +1,36 @@
+// Copyright 2019 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_PROC_H_
+#define DAWN_DAWN_PROC_H_
+
+#include "dawn/dawn.h"
+#include "dawn/dawn_proc_table.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Sets the static proctable used by libdawn_proc to implement the Dawn entrypoints. Passing NULL
+// for `procs` sets up the null proctable that contains only null function pointers. It is the
+// default value of the proctable. Setting the proctable back to null is good practice when you
+// are done using libdawn_proc since further usage will cause a segfault instead of calling an
+// unexpected function.
+DAWN_EXPORT void dawnProcSetProcs(const DawnProcTable* procs);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // DAWN_DAWN_PROC_H_

src/include/dawn_native/DawnNative.h

@@ -16,6 +16,7 @@
 #define DAWNNATIVE_DAWNNATIVE_H_
 
 #include <dawn/dawn.h>
+#include <dawn/dawn_proc_table.h>
 #include <dawn_native/dawn_native_export.h>
 
 #include <string>

src/include/dawn_wire/WireClient.h

@@ -15,10 +15,11 @@
 #ifndef DAWNWIRE_WIRECLIENT_H_
 #define DAWNWIRE_WIRECLIENT_H_
 
-#include <memory>
-
+#include "dawn/dawn_proc_table.h"
 #include "dawn_wire/Wire.h"
 
+#include <memory>
+
 namespace dawn_wire {
 
     namespace client {

src/include/dawn_wire/WireServer.h

@@ -19,6 +19,8 @@
 
 #include "dawn_wire/Wire.h"
 
+struct DawnProcTable;
+
 namespace dawn_wire {
 
     namespace server {