Add --disable-panels for blocking all extensions from using panels.

chrome/browser/ui/panels/panel_manager.cc

Index: chrome/browser/ui/panels/panel_manager.cc
diff --git a/chrome/browser/ui/panels/panel_manager.cc b/chrome/browser/ui/panels/panel_manager.cc
index ef8a026c3a26626f638c9d19e6d7ae0494d61509..06edbb9d9932c807637ddbdbf7666b2e5b84896f 100644
--- a/chrome/browser/ui/panels/panel_manager.cc
+++ b/chrome/browser/ui/panels/panel_manager.cc
@@ -105,6 +105,11 @@ void PanelManager::SetDisplaySettingsProviderForTesting(
 
 // static
 bool PanelManager::ShouldUsePanels(const std::string& extension_id) {
+  // If --disable-panels is on, never use panels.
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kDisablePanels))
+    return false;
+
   // If --enable-panels is on, always use panels.
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kEnablePanels))