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>

generator/templates/opengl/OpenGLFunctionsBase.h

@@ -18,7 +18,7 @@
 #include "dawn_native/Error.h"
 #include "dawn_native/opengl/opengl_platform.h"
 
-namespace dawn_native { namespace opengl {
+namespace dawn_native::opengl {
     using GetProcAddress = void* (*) (const char*);
 
     struct OpenGLFunctionsBase {
@@ -40,6 +40,6 @@ namespace dawn_native { namespace opengl {
         MaybeError LoadProc(GetProcAddress getProc, T* memberProc, const char* name);
     };
 
-}}  // namespace dawn_native::opengl
+}  // namespace dawn_native::opengl
 
 #endif // DAWNNATIVE_OPENGL_OPENGLFUNCTIONSBASE_H_