Replace NULL with nullptr or null in webrtc/base/.

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Commit-Position: refs/heads/master@{#16878}
diff --git a/webrtc/base/socketaddress_unittest.cc b/webrtc/base/socketaddress_unittest.cc
index e235447..072918e 100644
--- a/webrtc/base/socketaddress_unittest.cc
+++ b/webrtc/base/socketaddress_unittest.cc
@@ -237,7 +237,7 @@
   EXPECT_FALSE(SocketAddressFromSockAddrStorage(addr_storage, &addr));
   EXPECT_EQ(from, addr);
 
-  EXPECT_FALSE(SocketAddressFromSockAddrStorage(addr_storage, NULL));
+  EXPECT_FALSE(SocketAddressFromSockAddrStorage(addr_storage, nullptr));
 }
 
 bool AreEqual(const SocketAddress& addr1,