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

tools/gn/filesystem_utils_unittest.cc

Index: tools/gn/filesystem_utils_unittest.cc
diff --git a/tools/gn/filesystem_utils_unittest.cc b/tools/gn/filesystem_utils_unittest.cc
index bbf33369507a08810d84346b2e35cf182c1bd4ee..d398366e69c6f8e27536d954d2167ec32ebb0b42 100644
--- a/tools/gn/filesystem_utils_unittest.cc
+++ b/tools/gn/filesystem_utils_unittest.cc
@@ -97,25 +97,25 @@ TEST(FilesystemUtils, EnsureStringIsInOutputDir) {
 
   // Some outside.
   Err err;
-  EXPECT_FALSE(EnsureStringIsInOutputDir(output_dir, "//foo", NULL, &err));
+  EXPECT_FALSE(EnsureStringIsInOutputDir(output_dir, "//foo", nullptr, &err));
   EXPECT_TRUE(err.has_error());
   err = Err();
-  EXPECT_FALSE(EnsureStringIsInOutputDir(output_dir, "//out/Debugit", NULL,
-                                         &err));
+  EXPECT_FALSE(
+      EnsureStringIsInOutputDir(output_dir, "//out/Debugit", nullptr, &err));
   EXPECT_TRUE(err.has_error());
 
   // Some inside.
   err = Err();
-  EXPECT_TRUE(EnsureStringIsInOutputDir(output_dir, "//out/Debug/", NULL,
-                                        &err));
+  EXPECT_TRUE(
+      EnsureStringIsInOutputDir(output_dir, "//out/Debug/", nullptr, &err));
   EXPECT_FALSE(err.has_error());
-  EXPECT_TRUE(EnsureStringIsInOutputDir(output_dir, "//out/Debug/foo", NULL,
-                                        &err));
+  EXPECT_TRUE(
+      EnsureStringIsInOutputDir(output_dir, "//out/Debug/foo", nullptr, &err));
   EXPECT_FALSE(err.has_error());
 
   // Pattern but no template expansions are allowed.
   EXPECT_FALSE(EnsureStringIsInOutputDir(output_dir, "{{source_gen_dir}}",
-                                         NULL, &err));
+                                         nullptr, &err));
   EXPECT_TRUE(err.has_error());
 }