OpenGL: Implement the backend connection and adapter.

The OpenGL backend can't gather discover default adapters because it
needs getProc to do anything so we add DiscoverAdapters method to
Instance that takes backend-specific options.

dawn_native::opengl::CreateDevice is removed in favor of the adapter
path so OpenGLBinding is modified to create an instance locally. This is
only temporary until all backends support adapters, at which point a lot
of *Binding code will be factored.

Also contains a small fix for Result<T, E> with movable types.

BUG=dawn:29

Change-Id: I4eb3d4a14a871af73e1872132aff72b45e5fe566
Reviewed-on: https://dawn-review.googlesource.com/c/3663
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/BackendConnection.h

@@ -36,6 +36,10 @@ namespace dawn_native {
         // options (such as debug adapters, custom driver libraries, etc.)
         virtual std::vector<std::unique_ptr<AdapterBase>> DiscoverDefaultAdapters() = 0;
 
+        // Returns new adapters created with the backend-specific options.
+        virtual ResultOrError<std::vector<std::unique_ptr<AdapterBase>>> DiscoverAdapters(
+            const AdapterDiscoveryOptionsBase* options);
+
       private:
         InstanceBase* mInstance = nullptr;
         BackendType mType;