Fix 873811 by guarding v8_session_ for calls via V8Inspector::Channel.

v8_session_ starts out being a nullptr, and the constructor
of InspectorSession initializes it. However, this initialization
involves calling inspector->connect which calls the inspector
session (this) with sendResponse / flushProtocolNotifications.

This PR tries to resolve it by guarding v8_session_ for the
V8Inspector::Channel methods.

Bug: 873811
Change-Id: I8142eb61f9a8f6fb003b9baa0b6222730234ccae
Reviewed-on: https://chromium-review.googlesource.com/1235054Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592881}

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

@@ -49,8 +49,12 @@ InspectorSession::InspectorSession(
       v8_session_state_(kV8StateKey),
       v8_session_state_json_(&v8_session_state_, /*default_value=*/String()) {
   v8_session_state_.InitFrom(&session_state_);
+
+  // inspector->connect may result in calls to |this| against the
+  // V8Inspector::Channel interface for receiving responses / notifications,
+  // while v8_session_ is still nullptr.
   v8_session_ =
-      inspector->connect(context_group_id, this,
+      inspector->connect(context_group_id, /*channel*/ this,
                          ToV8InspectorStringView(v8_session_state_json_.Get()));
 }
 
@@ -144,7 +148,8 @@ void InspectorSession::SendProtocolResponse(int call_id,
   if (disposed_)
     return;
   flushProtocolNotifications();
-  v8_session_state_json_.Set(ToCoreString(v8_session_->stateJSON()));
+  if (v8_session_)
+    v8_session_state_json_.Set(ToCoreString(v8_session_->stateJSON()));
   client_->SendProtocolResponse(session_id_, call_id, message,
                                 session_state_.TakeUpdates());
 }
@@ -210,7 +215,8 @@ void InspectorSession::flushProtocolNotifications() {
     agents_[i]->FlushPendingProtocolNotifications();
   if (!notification_queue_.size())
     return;
-  v8_session_state_json_.Set(ToCoreString(v8_session_->stateJSON()));
+  if (v8_session_)
+    v8_session_state_json_.Set(ToCoreString(v8_session_->stateJSON()));
   for (wtf_size_t i = 0; i < notification_queue_.size(); ++i) {
     client_->SendProtocolNotification(session_id_,
                                       notification_queue_[i]->Serialize(),