'Add extension' bubble to use the dialog accessible role.

This bubble is modal: it takes focus when shown up and does not let
users manipulate other UI until having explicitly dealt with it. For
this reason, the dialog role is more appropriate.

As a result of this change, the bug where the bubble content was not
always read is gone.

The deleted code was supposed to fix crbug.com/849910, but now this
bubble is properly announced by JAWS 2020 in addition to NVDA.

Bug: 1123107
Change-Id: I371523a4384b7338e35ba30f897bade1d3ba2ee5
AX-relnotes: always announce add extension bubble correctly.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461276
Commit-Queue: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816510}

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

@@ -421,10 +421,6 @@ void ExtensionInstallDialogView::OnShutdown(
   CloseDialog();
 }
 
-ax::mojom::Role ExtensionInstallDialogView::GetAccessibleWindowRole() {
-  return ax::mojom::Role::kAlertDialog;
-}
-
 base::string16 ExtensionInstallDialogView::GetAccessibleWindowTitle() const {
   return title_;
 }

chrome/browser/ui/views/extensions/extension_install_dialog_view.h

@@ -69,7 +69,6 @@ class ExtensionInstallDialogView
   void OnShutdown(extensions::ExtensionRegistry* registry) override;
 
   // views::WidgetDelegate:
-  ax::mojom::Role GetAccessibleWindowRole() override;
   base::string16 GetAccessibleWindowTitle() const override;
   ui::ModalType GetModalType() const override;