MetalBinding: query the MTLDevice instead of creating it.

With adapters the Metal backend will be in charge of creating the
MTLDevice so we remove this responsibility from the bindings.

BUG=dawn:29

Change-Id: Id7b9e5f6249963e2b87a91242a18119ba8d11c13
Reviewed-on: https://dawn-review.googlesource.com/c/3661
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/DeviceMTL.h

@@ -34,7 +34,7 @@ namespace dawn_native { namespace metal {
 
     class Device : public DeviceBase {
       public:
-        Device(id<MTLDevice> mtlDevice);
+        Device();
         ~Device();
 
         CommandBufferBase* CreateCommandBuffer(CommandBufferBuilder* builder) override;

src/dawn_native/metal/DeviceMTL.mm

@@ -122,8 +122,13 @@ namespace dawn_native { namespace metal {
         }
     }  // anonymous namespace
 
-    dawnDevice CreateDevice(id<MTLDevice> metalDevice) {
-        return reinterpret_cast<dawnDevice>(new Device(metalDevice));
+    dawnDevice CreateDevice() {
+        return reinterpret_cast<dawnDevice>(new Device());
+    }
+
+    id<MTLDevice> GetMetalDevice(dawnDevice cDevice) {
+        Device* device = reinterpret_cast<Device*>(cDevice);
+        return device->GetMTLDevice();
     }
 
     BackendConnection* Connect(InstanceBase* instance) {
@@ -132,8 +137,8 @@ namespace dawn_native { namespace metal {
 
     // Device
 
-    Device::Device(id<MTLDevice> mtlDevice)
-        : mMtlDevice(mtlDevice),
+    Device::Device()
+        : mMtlDevice(MTLCreateSystemDefaultDevice()),
           mMapTracker(new MapRequestTracker(this)),
           mResourceUploader(new ResourceUploader(this)) {
         [mMtlDevice retain];

src/include/dawn_native/MetalBackend.h

@@ -23,7 +23,8 @@
 #import <QuartzCore/CAMetalLayer.h>
 
 namespace dawn_native { namespace metal {
-    DAWN_NATIVE_EXPORT dawnDevice CreateDevice(id<MTLDevice> metalDevice);
+    DAWN_NATIVE_EXPORT dawnDevice CreateDevice();
+    DAWN_NATIVE_EXPORT id<MTLDevice> GetMetalDevice(dawnDevice device);
 }}  // namespace dawn_native::metal
 
 #endif  // DAWNNATIVE_METALBACKEND_H_

src/utils/MetalBinding.mm

@@ -111,9 +111,11 @@ namespace utils {
         void SetupGLFWWindowHints() override {
             glfwWindowHint(GLFW_CLIENT_API, GLFW_NO_API);
         }
+
         dawnDevice CreateDevice() override {
-            mMetalDevice = MTLCreateSystemDefaultDevice();
-            return dawn_native::metal::CreateDevice(mMetalDevice);
+            dawnDevice device = dawn_native::metal::CreateDevice();
+            mMetalDevice = dawn_native::metal::GetMetalDevice(device);
+            return device;
         }
 
         uint64_t GetSwapChainImplementation() override {