Our validation wouldn't correctly handle single-element arrays because it
was checking size > 1 instead of isArray().  This caused issues in
ProgramGL on some drivers that optimized the array size (AMD) if some elements
were unused.

BUG=angleproject:882

Change-Id: I417d13cd86380e2c6caa688f6398709a74692e21
Reviewed-on: https://chromium-review.googlesource.com/289201Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>