Use std::nullptr_t in place of nullptr_t.

(It's not clear to me why only some projects/compilers are complaining
but other parts of the code are using the explicit namespace.)

Change-Id: I24b0c3b8104932b4ec82a25f5eb96c08abff87b5
Reviewed-on: https://dawn-review.googlesource.com/c/2901
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/generator/templates/apicpp.h b/generator/templates/apicpp.h
index 87bd71b..f1d732c 100644
--- a/generator/templates/apicpp.h
+++ b/generator/templates/apicpp.h
@@ -97,8 +97,8 @@
                 return static_cast<Derived&>(*this);
             }
 
-            ObjectBase(nullptr_t) {}
-            Derived& operator=(nullptr_t) {
+            ObjectBase(std::nullptr_t) {}
+            Derived& operator=(std::nullptr_t) {
                 if (mHandle != nullptr) {
                     Derived::DawnRelease(mHandle);
                     mHandle = nullptr;