Reset event bindings in AppBannerManager::OnInstall.

This addresses a bug where sites using the beforeinstallpromptevent to
implement an install button could have users trigger a do-nothing
install prompt if the user first used the menu item "Add to Shelf" to
install, and then pressed the beforeinstallprompt-connected button.

Resetting the event binding when the OnInstall() method is triggered
means that when an install is started, all outstanding
beforeinstallpromptevents are voided, fixing the bug.

BUG=823602

Change-Id: Ib2c747c74f7083a7663580ed37da7a319d9a9c60
Reviewed-on: https://chromium-review.googlesource.com/970050
Reviewed-by: Matt Giuca <[email protected]>
Commit-Queue: Dominick Ng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#544317}

chrome/browser/banners/app_banner_manager.cc

@@ -191,6 +191,10 @@ void AppBannerManager::OnInstall(bool is_native,
       mojo::MakeRequest(&installation_service));
   DCHECK(installation_service);
   installation_service->OnInstall();
+
+  // We've triggered an installation, so reset bindings to ensure that any
+  // existing beforeinstallprompt events cannot trigger add to home screen.
+  ResetBindings();
 }
 
 void AppBannerManager::SendBannerAccepted() {