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/dawn_platform/tracing/EventTracer.cpp

@@ -16,7 +16,7 @@
 #include "common/Assert.h"
 #include "dawn_platform/DawnPlatform.h"
 
-namespace dawn_platform { namespace tracing {
+namespace dawn_platform::tracing {
 
     const unsigned char* GetTraceCategoryEnabledFlag(Platform* platform, TraceCategory category) {
         static unsigned char disabled = 0;
@@ -55,4 +55,4 @@ namespace dawn_platform { namespace tracing {
         return static_cast<TraceEventHandle>(0);
     }
 
-}}  // namespace dawn_platform::tracing
+}  // namespace dawn_platform::tracing

src/dawn_platform/tracing/TraceEvent.h

@@ -754,7 +754,7 @@
 #define TRACE_VALUE_TYPE_STRING (static_cast<unsigned char>(6))
 #define TRACE_VALUE_TYPE_COPY_STRING (static_cast<unsigned char>(7))
 
-namespace dawn_platform { namespace TraceEvent {
+namespace dawn_platform::TraceEvent {
 
     // Specify these values when the corresponding argument of addTraceEvent is not
     // used.
@@ -986,6 +986,6 @@ namespace dawn_platform { namespace TraceEvent {
             Data m_data;
         };
 
-}}  // namespace dawn_platform::TraceEvent
+}  // namespace dawn_platform::TraceEvent
 
 #endif  // DAWNPLATFORM_TRACING_TRACEEVENT_H_