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/bitbuffer_unittest.cc b/webrtc/base/bitbuffer_unittest.cc
index aa9a1e7..1614299 100644
--- a/webrtc/base/bitbuffer_unittest.cc
+++ b/webrtc/base/bitbuffer_unittest.cc
@@ -169,8 +169,8 @@
   // nicely.
 #if GTEST_HAS_DEATH_TEST
 #if !defined(WEBRTC_ANDROID)
-  // Passing a NULL out parameter is death.
-  EXPECT_DEATH(buffer.GetCurrentOffset(&byte_offset, NULL), "");
+  // Passing a null out parameter is death.
+  EXPECT_DEATH(buffer.GetCurrentOffset(&byte_offset, nullptr), "");
 #endif
 #endif
 }