New post-sideload UI:

chrome/common/extensions/feature_switch.cc

Index: chrome/common/extensions/feature_switch.cc
diff --git a/chrome/common/extensions/feature_switch.cc b/chrome/common/extensions/feature_switch.cc
index 0bf53f38e8b126cc36495319adf9ae41be02a6d5..eaa2716de28c3380ed645aa84bccf6de490ee9b1 100644
--- a/chrome/common/extensions/feature_switch.cc
+++ b/chrome/common/extensions/feature_switch.cc
@@ -30,7 +30,11 @@ class CommonSwitches {
             FeatureSwitch::DEFAULT_DISABLED),
         script_bubble(
             switches::kScriptBubble,
-            FeatureSwitch::DEFAULT_DISABLED) {
+            FeatureSwitch::DEFAULT_DISABLED),
+        external_extension_prompt(
+            switches::kExternalExtensionPrompt,
+            FeatureSwitch::DEFAULT_ENABLED)
+  {
 // Disabling easy off-store installation is not yet implemented for Aura. Not
 // sure what the Aura equivalent for this UI is.
 #if defined(USE_AURA)
@@ -48,6 +52,7 @@ class CommonSwitches {
   FeatureSwitch extensions_in_action_box;
   FeatureSwitch script_badges;
   FeatureSwitch script_bubble;
+  FeatureSwitch external_extension_prompt;
 };
 
 base::LazyInstance<CommonSwitches> g_common_switches =
@@ -71,6 +76,9 @@ FeatureSwitch* FeatureSwitch::script_badges() {
 FeatureSwitch* FeatureSwitch::script_bubble() {
   return &g_common_switches.Get().script_bubble;
 }
+FeatureSwitch* FeatureSwitch::external_extension_prompt() {
+  return &g_common_switches.Get().external_extension_prompt;
+}
 
 
 FeatureSwitch::ScopedOverride::ScopedOverride(FeatureSwitch* feature,