dawn_native: define exported functions in their own files.

This adds a new FooBackend.cpp file for each backend that defines
exactly the symbols that are exported by libdawn_native's headers. THis
will allow factoring most of the compilation of dawn_native between the
shared and static library variants.

BUG=dawn:85

Change-Id: I69c808dac2fe5e8bb08356117e3997677787b08a
Reviewed-on: https://dawn-review.googlesource.com/c/3760
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/opengl/BackendGL.cpp

@@ -19,12 +19,6 @@
 
 namespace dawn_native { namespace opengl {
 
-    // Implementation of OpenGLBackend.h
-
-    AdapterDiscoveryOptions::AdapterDiscoveryOptions()
-        : AdapterDiscoveryOptionsBase(BackendType::OpenGL) {
-    }
-
     // The OpenGL backend's Adapter.
 
     class Adapter : public AdapterBase {

src/dawn_native/opengl/DeviceGL.cpp

@@ -17,7 +17,6 @@
 #include "dawn_native/BackendConnection.h"
 #include "dawn_native/BindGroup.h"
 #include "dawn_native/BindGroupLayout.h"
-#include "dawn_native/OpenGLBackend.h"
 #include "dawn_native/RenderPassDescriptor.h"
 #include "dawn_native/opengl/BufferGL.h"
 #include "dawn_native/opengl/CommandBufferGL.h"

src/dawn_native/opengl/OpenGLBackend.cpp

@@ -0,0 +1,28 @@
+// 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.
+
+// OpenGLBackend.cpp: contains the definition of symbols exported by OpenGLBackend.h so that they
+// can be compiled twice: once export (shared library), once not exported (static library)
+
+#include "dawn_native/OpenGLBackend.h"
+
+#include "dawn_native/opengl/DeviceGL.h"
+
+namespace dawn_native { namespace opengl {
+
+    AdapterDiscoveryOptions::AdapterDiscoveryOptions()
+        : AdapterDiscoveryOptionsBase(BackendType::OpenGL) {
+    }
+
+}}  // namespace dawn_native::opengl