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/compiler/translator/ParseContext.cpp b/src/compiler/translator/ParseContext.cpp
index f469746..ead6016 100644
--- a/src/compiler/translator/ParseContext.cpp
+++ b/src/compiler/translator/ParseContext.cpp
@@ -1536,7 +1536,7 @@
                                                  const TString *name,
                                                  const TSymbol *symbol)
 {
-    const TVariable *variable = NULL;
+    const TVariable *variable = nullptr;
 
     if (!symbol)
     {