Use comparison with 0 for explicit conversion to bool.

Bug: angleproject:3606
Change-Id: Ie0a76d7df829227c1376894535813b54e13491b4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2631689
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>