tools/gn: Pass clang-modernize -use-nullptr over it.

tools/gn/parse_tree_unittest.cc

Index: tools/gn/parse_tree_unittest.cc
diff --git a/tools/gn/parse_tree_unittest.cc b/tools/gn/parse_tree_unittest.cc
index 936947b21d43128657770524d762a93418ebfa52..baae36a30644a5ce29e7151c3943279da253262d 100644
--- a/tools/gn/parse_tree_unittest.cc
+++ b/tools/gn/parse_tree_unittest.cc
@@ -32,8 +32,9 @@ TEST(ParseTree, Accessor) {
 
   // Define a as a Scope. It should still fail because b isn't defined.
   err = Err();
-  setup.scope()->SetValue("a",
-      Value(NULL, scoped_ptr<Scope>(new Scope(setup.scope()))), NULL);
+  setup.scope()->SetValue(
+      "a", Value(nullptr, scoped_ptr<Scope>(new Scope(setup.scope()))),
+      nullptr);
   result = accessor.Execute(setup.scope(), &err);
   EXPECT_TRUE(err.has_error());
   EXPECT_EQ(Value::NONE, result.type());
@@ -41,8 +42,10 @@ TEST(ParseTree, Accessor) {
   // Define b, accessor should succeed now.
   const int64 kBValue = 42;
   err = Err();
-  setup.scope()->GetMutableValue("a", false)->scope_value()->SetValue(
-      "b", Value(NULL, kBValue), NULL);
+  setup.scope()
+      ->GetMutableValue("a", false)
+      ->scope_value()
+      ->SetValue("b", Value(nullptr, kBValue), nullptr);
   result = accessor.Execute(setup.scope(), &err);
   EXPECT_FALSE(err.has_error());
   ASSERT_EQ(Value::INTEGER, result.type());