Replace NULL with nullptr in //gpu

NULL blocks scoped_refptr to support nullptr, and eventually blocks the
removal of its implicit raw-pointer-to-scoped_refptr conversion.

This CL replaces all relevant NULL in //gpu with nullptr.

Tbr: [email protected]
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I6009249763ed14961bd5a725534419d2d0bf71f0
Reviewed-on: https://chromium-review.googlesource.com/1168960
Commit-Queue: Taiju Tsuiki <[email protected]>
Reviewed-by: Antoine Labour <[email protected]>
Cr-Commit-Position: refs/heads/master@{#582855}
139 files changed