[AppBanner] Blink implementation of an event when an install banner is shown.

public/web/WebLocalFrame.h

Index: public/web/WebLocalFrame.h
diff --git a/public/web/WebLocalFrame.h b/public/web/WebLocalFrame.h
index 3cc1018a189f682e0b16d40fa45145ddd509c339..f4786a2c5ee4483dbeaff7d8b2a3997fa52eb292 100644
--- a/public/web/WebLocalFrame.h
+++ b/public/web/WebLocalFrame.h
@@ -108,6 +108,14 @@ public:
     // selection to collapse. If the new extent is set to the same position as
     // the current base, this function will do nothing.
     virtual void moveRangeSelectionExtent(const WebPoint&, TextGranularity = CharacterGranularity) = 0;
+
+
+    // App banner -------------------------------------------------------------
+
+    // Request to show an application install banner for the given |platform|.
+    // The implementation can request the embedder to cancel the call by setting
+    // |cancel| to true.
+    virtual void willShowInstallBannerPrompt(const WebString& platform, bool* cancel) = 0;
 };
 
 } // namespace blink