Replace NULL with nullptr in all C++ files.

webrtc/base/stringutils_unittest.cc

Index: webrtc/base/stringutils_unittest.cc
diff --git a/webrtc/base/stringutils_unittest.cc b/webrtc/base/stringutils_unittest.cc
index 5e034dd1cdaec804b65f8afc62f61205039d60a0..421acb0b714e45de444bbc37c03fbdb1c8770a73 100644
--- a/webrtc/base/stringutils_unittest.cc
+++ b/webrtc/base/stringutils_unittest.cc
@@ -33,13 +33,13 @@ TEST(string_matchTest, Matches) {
 
 // Tests for ascii_string_compare().
 
-// Tests NULL input.
+// Tests null input.
 TEST(ascii_string_compareTest, NullInput) {
   // The following results in an access violation in
   // ascii_string_compare.  Is this a bug or by design?  stringutils.h
   // should document the expected behavior in this case.
 
-  // EXPECT_EQ(0, ascii_string_compare(NULL, NULL, 1, identity));
+  // EXPECT_EQ(0, ascii_string_compare(null, null, 1, identity));
 }
 
 // Tests comparing two strings of different lengths.