Make PWA install confirmation have no initially focused button

This CL changes the default action in a PWA install confirmation dialog
to be nothing. Before this change it was cancel.
This is in the interest of consistency with other situations where we
don't want "do security sensitive action" to be the default and for
accessibility concerns.

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=392968&signed_aid=sk4b6UZkAkZ9U6T9qqwovw==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=395017&signed_aid=jecG6nW4xOnmcpJS4uDMYA==&inline=1

Bug: 957848
Change-Id: Id2b39cac3685891db5605b12aeaa2d069236316a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626842
Commit-Queue: Jay Harris <harrisjay@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Jay Harris <harrisjay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662978}

chrome/browser/ui/views/extensions/pwa_confirmation.cc

@@ -128,7 +128,7 @@ base::string16 PWAConfirmation::GetDialogButtonLabel(ui::DialogButton button) {
 
 views::View* PWAConfirmation::GetInitiallyFocusedView(
     views::DialogDelegateView* dialog) {
-  return dialog->GetDialogClientView()->cancel_button();
+  return nullptr;
 }
 
 void PWAConfirmation::Accept() {