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/Context.cpp b/src/libANGLE/Context.cpp
index d44ea66..178eeef 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -639,7 +639,7 @@
 {
     GLuint handle = mQueryHandleAllocator.allocate();
 
-    mQueryMap[handle] = NULL;
+    mQueryMap[handle] = nullptr;
 
     return handle;
 }