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/tests/gl_tests/MipmapTest.cpp b/src/tests/gl_tests/MipmapTest.cpp
index d2f7ff9..b1b2dab 100644
--- a/src/tests/gl_tests/MipmapTest.cpp
+++ b/src/tests/gl_tests/MipmapTest.cpp
@@ -158,7 +158,8 @@
 
         // Initialize the texture2D to be empty, and don't use mips.
         glBindTexture(GL_TEXTURE_2D, mTexture2D);
-        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, getWindowWidth(), getWindowHeight(), 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
+        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, getWindowWidth(), getWindowHeight(), 0, GL_RGB,
+                     GL_UNSIGNED_BYTE, nullptr);
         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 
@@ -459,7 +460,8 @@
     int n = 1;
     while (getWindowWidth() / (1U << n) >= 1)
     {
-        glTexImage2D(GL_TEXTURE_2D, n, GL_RGB, getWindowWidth() / (1U << n), getWindowWidth() / (1U << n), 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
+        glTexImage2D(GL_TEXTURE_2D, n, GL_RGB, getWindowWidth() / (1U << n),
+                     getWindowWidth() / (1U << n), 0, GL_RGB, GL_UNSIGNED_BYTE, nullptr);
         ASSERT_GL_NO_ERROR();
         n+=1;
     }