Replace NULL with nullptr in all C++ files.

webrtc/modules/video_coding/receiver_unittest.cc

Index: webrtc/modules/video_coding/receiver_unittest.cc
diff --git a/webrtc/modules/video_coding/receiver_unittest.cc b/webrtc/modules/video_coding/receiver_unittest.cc
index 1fdbefd9982388686e5f1ed8f2ea2b15607e5147..d3ebe2d1abaced0fe0701af256c2fdbbfee1855f 100644
--- a/webrtc/modules/video_coding/receiver_unittest.cc
+++ b/webrtc/modules/video_coding/receiver_unittest.cc
@@ -439,7 +439,7 @@ TEST_F(VCMReceiverTimingTest, FrameForDecoding) {
     // max_wait_time.
     // In the case that we did not get a frame, it should have been waiting for
     // exactly max_wait_time. (By the testing samples we constructed above, we
-    // are sure there is no timing error, so the only case it returns with NULL
+    // are sure there is no timing error, so the only case it returns with null
     // is that it runs out of time.)
     if (frame) {
       receiver_.ReleaseFrame(frame);