Use C++17 nested namespaces instead of manually nesting them.

This was done with these two commands and a couple manual fixups for
namespaces that had more than one space in the comment in the closing
brace, as well as vulkan_platform.h

git grep -l "namespace .* { namespace " | xargs sed -i "" "s/namespace \(.*\) { namespace /namespace \1::/"
git grep -l "}}  // namespace" | xargs sed -i "" "s%}}  // namespace%}  // namespace%"

Bug: dawn:824
Change-Id: I6f448b820c12fc1004ea5270bf8e1f466b0c0aab
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/75400
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/include/dawn_native/D3D12Backend.h

@@ -28,7 +28,7 @@
 struct ID3D12Device;
 struct ID3D12Resource;
 
-namespace dawn_native { namespace d3d12 {
+namespace dawn_native::d3d12 {
 
     class D3D11on12ResourceCache;
 
@@ -101,6 +101,6 @@ namespace dawn_native { namespace d3d12 {
         Microsoft::WRL::ComPtr<IDXGIAdapter> dxgiAdapter;
     };
 
-}}  // namespace dawn_native::d3d12
+}  // namespace dawn_native::d3d12
 
 #endif  // DAWNNATIVE_D3D12BACKEND_H_

src/include/dawn_native/MetalBackend.h

@@ -32,7 +32,7 @@ typedef __IOSurface* IOSurfaceRef;
 #    import <Metal/Metal.h>
 #endif  //__OBJC__
 
-namespace dawn_native { namespace metal {
+namespace dawn_native::metal {
 
     struct DAWN_NATIVE_EXPORT AdapterDiscoveryOptions : public AdapterDiscoveryOptionsBase {
         AdapterDiscoveryOptions();
@@ -56,14 +56,14 @@ namespace dawn_native { namespace metal {
     // scheduled could lead to races in who gets scheduled first and incorrect rendering.
     DAWN_NATIVE_EXPORT void WaitForCommandsToBeScheduled(WGPUDevice device);
 
-}}  // namespace dawn_native::metal
+}  // namespace dawn_native::metal
 
 #ifdef __OBJC__
-namespace dawn_native { namespace metal {
+namespace dawn_native::metal {
 
     DAWN_NATIVE_EXPORT id<MTLDevice> GetMetalDevice(WGPUDevice device);
 
-}}      // namespace dawn_native::metal
+}  // namespace dawn_native::metal
 #endif  // __OBJC__
 
 #pragma clang diagnostic pop

src/include/dawn_native/NullBackend.h

@@ -18,8 +18,8 @@
 #include <dawn/dawn_wsi.h>
 #include <dawn_native/DawnNative.h>
 
-namespace dawn_native { namespace null {
+namespace dawn_native::null {
     DAWN_NATIVE_EXPORT DawnSwapChainImplementation CreateNativeSwapChainImpl();
-}}  // namespace dawn_native::null
+}  // namespace dawn_native::null
 
 #endif  // DAWNNATIVE_NULLBACKEND_H_

src/include/dawn_native/OpenGLBackend.h

@@ -20,7 +20,7 @@ typedef void* EGLImage;
 #include <dawn/dawn_wsi.h>
 #include <dawn_native/DawnNative.h>
 
-namespace dawn_native { namespace opengl {
+namespace dawn_native::opengl {
 
     struct DAWN_NATIVE_EXPORT AdapterDiscoveryOptions : public AdapterDiscoveryOptionsBase {
         AdapterDiscoveryOptions();
@@ -50,6 +50,6 @@ namespace dawn_native { namespace opengl {
     DAWN_NATIVE_EXPORT WGPUTexture
     WrapExternalEGLImage(WGPUDevice device, const ExternalImageDescriptorEGLImage* descriptor);
 
-}}  // namespace dawn_native::opengl
+}  // namespace dawn_native::opengl
 
 #endif  // DAWNNATIVE_OPENGLBACKEND_H_

src/include/dawn_native/VulkanBackend.h

@@ -22,7 +22,7 @@
 
 #include <vector>
 
-namespace dawn_native { namespace vulkan {
+namespace dawn_native::vulkan {
 
     DAWN_NATIVE_EXPORT VkInstance GetInstance(WGPUDevice device);
 
@@ -135,6 +135,6 @@ namespace dawn_native { namespace vulkan {
                                                   VkImageLayout desiredLayout,
                                                   ExternalImageExportInfoVk* info);
 
-}}  // namespace dawn_native::vulkan
+}  // namespace dawn_native::vulkan
 
 #endif  // DAWNNATIVE_VULKANBACKEND_H_