This improves perf because vertex data upload uses host visible memory,
and uncached memory can be slow.

Test:

Android on Windows with 1080Ti:
glmark2 [build] vbo=false 265 -> 3900 fps

Bug: angleproject:5515
Change-Id: Ib75582bbdb4ca4a22d3a121c7b477a23a8a809ef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2615742
Commit-Queue: Lingfeng Yang <lfy@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>