Implement experimental app banner flow.

chrome/browser/banners/app_banner_manager.h

Index: chrome/browser/banners/app_banner_manager.h
diff --git a/chrome/browser/banners/app_banner_manager.h b/chrome/browser/banners/app_banner_manager.h
index d14251b7dc4aeaf9c35048e288ad112c24d9f9f9..ff898cc6bd0f4726dc5d8d936942ee1d4663d293 100644
--- a/chrome/browser/banners/app_banner_manager.h
+++ b/chrome/browser/banners/app_banner_manager.h
@@ -251,13 +251,16 @@ class AppBannerManager : public content::WebContentsObserver,
  private:
   friend class AppBannerManagerTest;
 
+  // Returns whether the new experimental flow and UI is enabled.
+  static bool IsExperimentalAppBannersEnabled();
+
   // Record that the banner could be shown at this point, if the triggering
   // heuristic allowed.
   void RecordCouldShowBanner();
 
-  // Creates a banner for the app. Overridden by subclasses as the infobar is
+  // Creates the app banner UI. Overridden by subclasses as the infobar is
   // platform-specific.
-  virtual void ShowBanner() = 0;
+  virtual void ShowBannerUI() = 0;
 
   // Called after the manager sends a message to the renderer regarding its
   // intention to show a prompt. The renderer will send a message back with the
@@ -265,6 +268,9 @@ class AppBannerManager : public content::WebContentsObserver,
   void OnBannerPromptReply(blink::mojom::AppBannerPromptReply reply,
                            const std::string& referrer);
 
+  // Does the non-platform specific parts of showing the app banner.
+  void ShowBanner();
+
   // blink::mojom::AppBannerService overrides.
   // Called when Blink has prevented a banner from being shown, and is now
   // requesting that it be shown later.