Replace NULL with nullptr in all C++ files.

webrtc/pc/srtpfilter_unittest.cc

Index: webrtc/pc/srtpfilter_unittest.cc
diff --git a/webrtc/pc/srtpfilter_unittest.cc b/webrtc/pc/srtpfilter_unittest.cc
index 32cd20ed6adb3980188b0336938753c4a893042b..e7e5d696f230828941aa9d5b0598d94c55fb0737 100644
--- a/webrtc/pc/srtpfilter_unittest.cc
+++ b/webrtc/pc/srtpfilter_unittest.cc
@@ -578,10 +578,10 @@ TEST_F(SrtpFilterTest, TestGetSendAuthParams) {
   EXPECT_TRUE(f1_.SetRtcpParams(rtc::SRTP_AES128_CM_SHA1_32, kTestKey1,
                                 kTestKeyLen, rtc::SRTP_AES128_CM_SHA1_32,
                                 kTestKey2, kTestKeyLen));
-  uint8_t* auth_key = NULL;
+  uint8_t* auth_key = nullptr;
   int auth_key_len = 0, auth_tag_len = 0;
   EXPECT_TRUE(f1_.GetRtpAuthParams(&auth_key, &auth_key_len, &auth_tag_len));
-  EXPECT_TRUE(auth_key != NULL);
+  EXPECT_TRUE(auth_key != nullptr);
   EXPECT_EQ(20, auth_key_len);
   EXPECT_EQ(4, auth_tag_len);
 }