Make libdawn_native a shared library.

The interface exposed by libdawn_native is declared in the new headers
living in src/include/dawn_native so that they both the users and the
libraries use the DAWN_NATIVE_EXPORT macros.

src/include/dawn_native/NullBackend.h

@@ -0,0 +1,25 @@
+// Copyright 2018 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 DAWNNATIVE_NULLBACKEND_H_
+#define DAWNNATIVE_NULLBACKEND_H_
+
+#include <dawn/dawn.h>
+#include <dawn_native/dawn_native_export.h>
+
+namespace dawn_native { namespace null {
+    DAWN_NATIVE_EXPORT void Init(dawnProcTable* procs, dawnDevice* device);
+}}  // namespace dawn_native::null
+
+#endif  // DAWNNATIVE_NULLBACKEND_H_