mash: Add a ui_devtools warning and TODO for local aura::Env usage.

Work will be needed to make ui_devtools work in multi-process Mash.
(otherwise it will only handle events sent to Chrome-owned windows)

Bug: 896977
Change-Id: Ie2487aed095ee9f62e2e9c85e56bc056b789e0ed
Reviewed-on: https://chromium-review.googlesource.com/c/1300056Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603711}

chrome/browser/ui/views/chrome_browser_main_extra_parts_views.cc

@@ -51,6 +51,10 @@
 #include "ui/base/l10n/l10n_util.h"
 #endif  // defined(OS_LINUX) && !defined(OS_CHROMEOS)
 
+#if defined(OS_CHROMEOS)
+#include "ui/base/ui_base_features.h"
+#endif
+
 ChromeBrowserMainExtraPartsViews::ChromeBrowserMainExtraPartsViews() {
 }
 
@@ -96,6 +100,15 @@ void ChromeBrowserMainExtraPartsViews::PreProfileInit() {
       network_context, switches::kEnableUiDevTools, 9223);
   if (devtools_server_) {
     auto dom_backend = std::make_unique<ui_devtools::DOMAgentAura>();
+#if defined(OS_CHROMEOS)
+    // OverlayAgentAura intends to handle input events targeting any UI surface,
+    // and so installs itself as a local aura::Env pre-target ui::EventHandler.
+    // In multi-process Mash, Chrome's local aura::Env can only handle events
+    // target Chrome's own aura::Windows, not those targeting Ash or mojo apps.
+    // TODO(crbug.com/896977): Init the devtools server in Ash on Chrome OS.
+    LOG_IF(WARNING, features::IsMultiProcessMash())
+        << "Chrome cannot handle Ash system ui and mojo app events in Mash.";
+#endif
     auto overlay_backend =
         std::make_unique<ui_devtools::OverlayAgentAura>(dom_backend.get());
     auto css_backend =