Add scoped autoreleasepool to tests and examples

This ensures ObjC objects do not leak. On non-Apple
platforms, the scope does nothing.

Fixed: dawn:546
Change-Id: Id43702e6536bf9cb37825a5449511e10cf0734f5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/55841
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

examples/ComputeBoids.cpp

@@ -15,6 +15,7 @@
 #include "SampleUtils.h"
 
 #include "utils/ComboRenderPipelineDescriptor.h"
+#include "utils/ScopedAutoreleasePool.h"
 #include "utils/SystemUtils.h"
 #include "utils/WGPUHelpers.h"
 
@@ -324,6 +325,7 @@ int main(int argc, const char* argv[]) {
     init();
 
     while (!ShouldQuit()) {
+        utils::ScopedAutoreleasePool pool;
         frame();
         utils::USleep(16000);
     }