Reintroduce --prompt-for-external-extensions flag on Chromium builds.

extensions/common/feature_switch.cc

Index: extensions/common/feature_switch.cc
diff --git a/extensions/common/feature_switch.cc b/extensions/common/feature_switch.cc
index 28c72c30c7ade824fd586113321d35e01ff44526..38b21b58d2420aa0676a69c0e4499250c730167e 100644
--- a/extensions/common/feature_switch.cc
+++ b/extensions/common/feature_switch.cc
@@ -20,11 +20,16 @@ class CommonSwitches {
       : easy_off_store_install(NULL, FeatureSwitch::DEFAULT_DISABLED),
         force_dev_mode_highlighting(switches::kForceDevModeHighlighting,
                                     FeatureSwitch::DEFAULT_DISABLED),
-        prompt_for_external_extensions(NULL,
+        prompt_for_external_extensions(
+#if defined(CHROMIUM_BUILD)
+            switches::kPromptForExternalExtensions,
+#else
+            NULL,
+#endif
 #if defined(OS_WIN)
-                                       FeatureSwitch::DEFAULT_ENABLED),
+            FeatureSwitch::DEFAULT_ENABLED),
 #else
-                                       FeatureSwitch::DEFAULT_DISABLED),
+            FeatureSwitch::DEFAULT_DISABLED),
 #endif
         error_console(switches::kErrorConsole, FeatureSwitch::DEFAULT_DISABLED),
         enable_override_bookmarks_ui(switches::kEnableOverrideBookmarksUI,