Refactors ANGLE Extensions Validation functions into a common format for
future autogeneration. Any writes to output parameters that occurred
within the ANGLE entry point have been moved into the corresponding
validation function.

Bug:angleproject:2263
Change-Id: I7a678310ad481c1379596a493fcb0b0383fcf3b8
Reviewed-on: https://chromium-review.googlesource.com/985244
Commit-Queue: Brandon1 Jones <brandon1.jones@intel.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>