UWP: Implement GetPathSeparatorForEnvironmentVar
UWP: Implement GetPathSeparatorForEnvironmentVar

Bug: angleproject:4142
Change-Id: Idb5e1b540d9ac7ab802357f247b41a6546173a35
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1925254
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle