Fixes buffer size calculation when uploading element buffer
to account for 4-byte (uint) indices.

Bug: angleproject:2902
Change-Id: Id70fc0fdffecd0b27995820bd0ad88e4cb61e013
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1648325Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>