Rename "callback" to "function pointer" category.
The function does not belongs to any class that has below two keys:
- `"returns"` a string that's the name of the return type
- `"args"` a **record**, so an array of **record members**
BUG=dawn:1201
Change-Id: I546301a19af9472501122e759c495fd761e27ed5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/71060
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>