This reduces the proliferation of small entry point files.

Bug: angleproject:2621
Change-Id: Ib882fd9e32e165279439d5d6cab7205eae0732ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2566592
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>