Adds missing includes for google3 roll.

Change-Id: Id99d168d3f127558179c7097af6575f07a260ed6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/134465
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Shrek Shao <shrekshao@google.com>

generator/templates/api_cpp.h

@@ -25,6 +25,8 @@
 #include "dawn/{{api}}_cpp_chained_struct.h"
 #include "dawn/EnumClassBitmasks.h"
 #include <cmath>
+#include <cstddef>
+#include <cstdint>
 
 namespace {{metadata.namespace}} {
 

generator/templates/api_cpp_chained_struct.h

@@ -20,6 +20,9 @@
 #ifndef {{API}}_CPP_CHAINED_STRUCT_H_
 #define {{API}}_CPP_CHAINED_STRUCT_H_
 
+#include <cstddef>
+#include <cstdint>
+
 // This header file declares the ChainedStruct structures separately from the {{metadata.api}}
 // headers so that dependencies can directly extend structures without including the larger header
 // which exposes capabilities that may require correctly set proc tables.