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

webrtc/base/bitbuffer.cc

Index: webrtc/base/bitbuffer.cc
diff --git a/webrtc/base/bitbuffer.cc b/webrtc/base/bitbuffer.cc
index fc8a899ad66b10b076c03147419a44afe3109103..4ea2b155a1da16f33fc203c3f9ad309462088058 100644
--- a/webrtc/base/bitbuffer.cc
+++ b/webrtc/base/bitbuffer.cc
@@ -202,8 +202,8 @@ bool BitBuffer::ReadSignedExponentialGolomb(int32_t* val) {
 
 void BitBuffer::GetCurrentOffset(
     size_t* out_byte_offset, size_t* out_bit_offset) {
-  RTC_CHECK(out_byte_offset != NULL);
-  RTC_CHECK(out_bit_offset != NULL);
+  RTC_CHECK(out_byte_offset != nullptr);
+  RTC_CHECK(out_bit_offset != nullptr);
   *out_byte_offset = byte_offset_;
   *out_bit_offset = bit_offset_;
 }