Code refactoring: replace NULL by nullptr for pointers.

This is the frist change to replace NULL by nullptr.
It handles the initialization and assignment for pointers.

BUG=angleproject:2001

Change-Id: I6d4bb198a72e38b867cd2f65a6e6f2f61339a0b5
Reviewed-on: https://chromium-review.googlesource.com/481600
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/validationES3.cpp b/src/libANGLE/validationES3.cpp
index 207b44c..fa85a9e 100644
--- a/src/libANGLE/validationES3.cpp
+++ b/src/libANGLE/validationES3.cpp
@@ -529,7 +529,7 @@
     //      sizes] are consistent with the values of the source buffer's [channel sizes]. Table 3.17
     //      is used if the FRAMEBUFFER_ATTACHMENT_ENCODING is LINEAR and table 3.18 is used if the
     //      FRAMEBUFFER_ATTACHMENT_ENCODING is SRGB.
-    const InternalFormat *sourceEffectiveFormat = NULL;
+    const InternalFormat *sourceEffectiveFormat = nullptr;
     if (readBufferHandle != 0)
     {
         // Not the default framebuffer, therefore the read buffer must be a user-created texture or