Remove usage of CHECK_NOTNULL

We want to switch to absl logging instead of glog.  gtest and ceres are
going there, and we already have absl as a dependency.  ABSL doesn't
have CHECK_NOTNULL, and we can move things over in an easier to review
fashion.

Change-Id: Ifd9a11ec34a2357cec43f88dba015db9c28ed2cf
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/network/web_proxy.cc b/aos/network/web_proxy.cc
index 15ebb66..6b71433 100644
--- a/aos/network/web_proxy.cc
+++ b/aos/network/web_proxy.cc
@@ -333,7 +333,7 @@
 
 std::shared_ptr<struct mbuf> Subscriber::NextBuffer(
     ChannelInformation *channel) {
-  CHECK_NOTNULL(channel);
+  CHECK(channel != nullptr);
   if (message_buffer_.empty()) {
     return nullptr;
   }
@@ -400,7 +400,7 @@
 }
 
 void Subscriber::SkipToLastMessage(ChannelInformation *channel) {
-  CHECK_NOTNULL(channel);
+  CHECK(channel != nullptr);
   if (message_buffer_.empty() ||
       channel->current_queue_index == message_buffer_.back().index) {
     return;