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/Context.cpp b/src/libANGLE/Context.cpp
index 178eeef..affc008 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -441,7 +441,7 @@
 
     for (auto &zeroTexture : mZeroTextures)
     {
-        zeroTexture.second.set(NULL);
+        zeroTexture.second.set(nullptr);
     }
     mZeroTextures.clear();
 
@@ -1184,7 +1184,7 @@
     gl::Error error = queryObject->end();
 
     // Always unbind the query, even if there was an error. This may delete the query object.
-    mGLState.setActiveQuery(target, NULL);
+    mGLState.setActiveQuery(target, nullptr);
 
     return error;
 }
@@ -1258,7 +1258,7 @@
 
     if (fence == mFenceNVMap.end())
     {
-        return NULL;
+        return nullptr;
     }
     else
     {
@@ -1272,7 +1272,7 @@
 
     if (query == mQueryMap.end())
     {
-        return NULL;
+        return nullptr;
     }
     else
     {