Prepratory clean up for WinUWP support.

Bug: angleproject:4182
Change-Id: I97cc6cb42a50c6b57ca7d375d1e0a6b3da7aa9d7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1998673
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>