Add --use-more-webui runtime flag to toggle WebUI replacements for native dialogs.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 79a5da26c2eeb6753468ab00d4e72addad6f9c6d..9f896e04584f422f84846e4b283df459f1c43e51 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1004,6 +1004,10 @@ const char kTryChromeAgain[]                = "try-chrome-again";
 // Runs un-installation steps that were done by chrome first-run.
 const char kUninstall[]                     = "uninstall";
 
+// Use WebUI versions of dialogs when available (rather than platform native
+// implementations).
+const char kUseMoreWebUI[]                  = "use-more-webui";
+
 // Use a pure Views implementation when available (rather rather than platform
 // native implementation such as GTK).
 const char kUsePureViews[]                  = "use-pure-views";