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>
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index f9c8c6a..657522c 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -757,7 +757,7 @@
                 }
                 // OES_depth_texture supports loading depth data and multiple levels,
                 // but ANGLE_depth_texture does not
-                if (pixels != NULL || level != 0)
+                if (pixels != nullptr || level != 0)
                 {
                     context->handleError(Error(GL_INVALID_OPERATION));
                     return false;