Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/screen_capturer_mac_unittest.cc

Index: webrtc/modules/desktop_capture/screen_capturer_mac_unittest.cc
diff --git a/webrtc/modules/desktop_capture/screen_capturer_mac_unittest.cc b/webrtc/modules/desktop_capture/screen_capturer_mac_unittest.cc
index 7133a9ebbcd21d75c7be3c6716ad22911154f988..40c2c4af84969794caeaab4d4d7eeb343c8d24e9 100644
--- a/webrtc/modules/desktop_capture/screen_capturer_mac_unittest.cc
+++ b/webrtc/modules/desktop_capture/screen_capturer_mac_unittest.cc
@@ -74,7 +74,7 @@ void ScreenCapturerMacTest::CaptureDoneCallback2(
 
   EXPECT_EQ(width, (*frame)->size().width());
   EXPECT_EQ(height, (*frame)->size().height());
-  EXPECT_TRUE((*frame)->data() != NULL);
+  EXPECT_TRUE((*frame)->data() != nullptr);
   // Depending on the capture method, the screen may be flipped or not, so
   // the stride may be positive or negative.
   EXPECT_EQ(static_cast<int>(sizeof(uint32_t) * width),