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/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_