[DevTools] Add a CHECK for io_sesssion_ in DevToolsSession::Detach().

This is as a debugging aid, to rule out that we're calling
io_session_->DeleteSoon twice from this method, for some reason.

Bug: 1004684
Change-Id: I13192c75a884f824c7d6ffd92cd17b834a34db6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809999
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697428}

third_party/blink/renderer/core/inspector/devtools_session.cc

@@ -218,6 +218,7 @@ void DevToolsSession::Detach() {
   agent_->sessions_.erase(this);
   binding_.Close();
   host_ptr_.reset();
+  CHECK(io_session_);
   io_session_->DeleteSoon();
   io_session_ = nullptr;
   agent_->probe_sink_->RemoveDevToolsSession(this);