This replaces are "IsUnsignedXXXSafe" family of methods.
Also add overflow checks to unpack block sizes.

BUG=angleproject:1397

Change-Id: Ib47be149b0486c70f795b0d0f8899441faac9340
Reviewed-on: https://chromium-review.googlesource.com/348062Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>