Use InterstitialPage through a delegate interface instead of deriving from it.

chrome/renderer/content_settings_observer.cc

Index: chrome/renderer/content_settings_observer.cc
===================================================================
--- chrome/renderer/content_settings_observer.cc	(revision 120733)
+++ chrome/renderer/content_settings_observer.cc	(working copy)
@@ -102,6 +102,7 @@
     // blocked plugin.
     IPC_MESSAGE_HANDLER_GENERIC(ChromeViewMsg_LoadBlockedPlugins,
                                 OnLoadBlockedPlugins(); handled = false)
+    IPC_MESSAGE_HANDLER(ChromeViewMsg_SetAsInterstitial, OnSetAsInterstitial)
     IPC_MESSAGE_UNHANDLED(handled = false)
   IPC_END_MESSAGE_MAP()
   return handled;
@@ -280,14 +281,14 @@
   DidBlockContentType(CONTENT_SETTINGS_TYPE_JAVASCRIPT, std::string());
 }
 
-void ContentSettingsObserver::SetAsInterstitial() {
-  is_interstitial_page_ = true;
-}
-
 void ContentSettingsObserver::OnLoadBlockedPlugins() {
   plugins_temporarily_allowed_ = true;
 }
 
+void ContentSettingsObserver::OnSetAsInterstitial() {
+  is_interstitial_page_ = true;
+}
+
 void ContentSettingsObserver::ClearBlockedContentSettings() {
   for (size_t i = 0; i < arraysize(content_blocked_); ++i)
     content_blocked_[i] = false;