dawn_native: Add Instance and Adapters

New objects are introduced to control what happens before device
creation in dawn_native:
 - Instance: a connection from the application to dawn_native that is
used for dependency injection and to discover adapters.
 - Adapters: represents the possibility of device creation for a specific
(GPU, backend) pair.
 - BackendConnection: an internal object that standardizes the interface
between the frontend and backends.

The BackendConnection interface is implemented for the Null backend and
stubbed out in other backends. This allows this change to port the
ValidationTests to use the new Instance and Adapters concept and deal
with other backends later.

BUG=dawn:29

Change-Id: I19719a9342b4af091accc0c02fb6b9697eadde7b
Reviewed-on: https://dawn-review.googlesource.com/c/3500
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/BackendConnection.h

@@ -0,0 +1,44 @@
+// 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_BACKENDCONNECTION_H_
+#define DAWNNATIVE_BACKENDCONNECTION_H_
+
+#include "dawn_native/Adapter.h"
+#include "dawn_native/DawnNative.h"
+
+namespace dawn_native {
+
+    // An common interface for all backends. Mostly used to create adapters for a particular
+    // backend.
+    class BackendConnection {
+      public:
+        BackendConnection(InstanceBase* instance, BackendType type);
+        virtual ~BackendConnection() = default;
+
+        BackendType GetType() const;
+        InstanceBase* GetInstance() const;
+
+        // Returns all the adapters for the system that can be created by the backend, without extra
+        // options (such as debug adapters, custom driver libraries, etc.)
+        virtual std::vector<std::unique_ptr<AdapterBase>> DiscoverDefaultAdapters() = 0;
+
+      private:
+        InstanceBase* mInstance = nullptr;
+        BackendType mType;
+    };
+
+}  // namespace dawn_native
+
+#endif  // DAWNNATIVE_BACKENDCONNECTION_H_