Add temporary CHECKs to BodyStreamBuffer

Add CHECKs to verify that |consumer_| is non-null in the BodyStreamBuffer
constructor. These will be removed once we find out whether it crashes
or not.

BUG=1007162

Change-Id: I6b511c4f2f7558d755f7db5f7bba4b452445a173
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1837445Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702351}

third_party/blink/renderer/core/fetch/body_stream_buffer.cc

@@ -107,10 +107,20 @@ BodyStreamBuffer::BodyStreamBuffer(ScriptState* script_state,
       consumer_(consumer),
       signal_(signal),
       made_from_readable_stream_(false) {
+  // This CHECK is temporary to track down the cause of
+  // https://crbug.com/1007162.
+  // TODO(ricea): Remove it once we know whether it crashes or not.
+  CHECK(consumer_);
+
   stream_ =
       ReadableStream::CreateWithCountQueueingStrategy(script_state_, this, 0);
   stream_broken_ = !stream_;
 
+  // This CHECK is temporary to track down the cause of
+  // https://crbug.com/1007162.
+  // TODO(ricea): Remove it once we know whether it crashes or not.
+  CHECK(consumer_);
+
   consumer_->SetClient(this);
   if (signal) {
     if (signal->aborted()) {