Make PWAInstallView::Update defensive towards the existence of tab helpers

This CL makes the PWA Omnibox install icon check the existence of
both the AppBannerManager and the WebAppTabHelperBase as the presence of
one is not 1:1 with the other.

Bug: 953409
Change-Id: I310aa4c1613eb1d4037cb417c7b5c0d51215eb35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585282
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654778}

chrome/browser/ui/views/page_action/pwa_install_view.cc

@@ -30,20 +30,17 @@ bool PwaInstallView::Update() {
   if (!web_contents)
     return false;
 
-  banners::AppBannerManager* manager =
-      banners::AppBannerManager::FromWebContents(web_contents);
+  auto* manager = banners::AppBannerManager::FromWebContents(web_contents);
   // May not be present e.g. in incognito mode.
   if (!manager)
     return false;
 
-  bool is_installable = manager->IsProbablyInstallable();
-  bool is_installed =
-      web_app::WebAppTabHelperBase::FromWebContents(web_contents)
-          ->HasAssociatedApp();
-  bool show_install_button = is_installable && !is_installed;
-  // TODO(crbug.com/907351): When installability is unknown and we're still in
-  // the scope of a previously-determined installable site, display it as still
-  // being installable.
+  bool is_probably_installable = manager->IsProbablyInstallable();
+  auto* tab_helper =
+      web_app::WebAppTabHelperBase::FromWebContents(web_contents);
+  bool is_installed = tab_helper && tab_helper->HasAssociatedApp();
+
+  bool show_install_button = is_probably_installable && !is_installed;
 
   if (show_install_button && manager->MaybeConsumeInstallAnimation())
     AnimateIn(base::nullopt);