Refactoring: replace NULL by nullptr for pointers (3rd CL).

This CL mainly handles passing/returning NULL to/from a function.

BUG=angleproject:2001

Change-Id: I34802f792e710e3d7ff697cbe4701dc1bf5ab009
Reviewed-on: https://chromium-review.googlesource.com/485060
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/validationES.cpp b/src/libANGLE/validationES.cpp
index e2c7af7..c118d8a 100644
--- a/src/libANGLE/validationES.cpp
+++ b/src/libANGLE/validationES.cpp
@@ -3785,7 +3785,7 @@
         return false;
     }
 
-    if (!program->validateSamplers(NULL, context->getCaps()))
+    if (!program->validateSamplers(nullptr, context->getCaps()))
     {
         context->handleError(Error(GL_INVALID_OPERATION));
         return false;