Not all call sites were checking the return value of
MemoryBuffer::resize, mark the return value as NO_DISCARD and fix all
the warnings.

BUG=chromium:1030835

Change-Id: I762796e3d11efc131a814069d78a195b0d4c9f8f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2028151
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>