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 db5d74edc68caf18ef1ac65cd554b705475b05b4..49fbfd90d3c4673ce6c6b8fef05078ac6ea98a44 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1013,6 +1013,10 @@ 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";