Add missing function prototypes to ANGLE ext headers. Bug: angleproject:7752 Change-Id: I316e181c61fe1daf3611c58b4db0321678d9eba2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3963365 Reviewed-by: Cody Northrop <cnorthrop@google.com>