[DevTools] Prevent crash when auto-attaching with flatten to frames

We are not ready for flatten in frames - the TargetRegistry is null.

Bug: 890760
Change-Id: I8bbffad4f74ffa12f371ce7feef32cae3d6f6304
Reviewed-on: https://chromium-review.googlesource.com/1255122Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595516}

content/browser/devtools/protocol/target_handler.cc

@@ -549,6 +549,10 @@ Response TargetHandler::SetDiscoverTargets(bool discover) {
 Response TargetHandler::SetAutoAttach(bool auto_attach,
                                       bool wait_for_debugger_on_start,
                                       Maybe<bool> flatten) {
+  if (flatten.fromMaybe(false) && !target_registry_) {
+    return Response::InvalidParams(
+        "Will only provide flatten access for browser endpoint");
+  }
   flatten_auto_attach_ = flatten.fromMaybe(false);
   auto_attacher_.SetAutoAttach(auto_attach, wait_for_debugger_on_start);
   if (!auto_attacher_.ShouldThrottleFramesNavigation())