Refactoring: replace NULL by nullptr for pointers (2nd CL). This CL mainly handles the pointer comparisons (== or !=). BUG=angleproject:2001 Change-Id: I25ac3b61032e7ad91459a1c6541cadc87cf9b160 Reviewed-on: https://chromium-review.googlesource.com/483935 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>