Change app dialogs back to the system modal.

App dialogs are all system modal to block the user switch to other
tasks. Well CL:2398943 change it to the window modal due to the desktop
chrome request. App dialogs are only used for Chrome OS, so they should
be the system modal.

BUG=1163638

Change-Id: I9f32a8c404a13321666a42be61013a3a096570ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617173
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841914}

chrome/browser/ui/views/apps/app_dialog/app_dialog_view.cc

@@ -17,7 +17,7 @@ AppDialogView::AppDialogView(const gfx::ImageSkia& image)
   SetIcon(image);
   SetShowIcon(true);
   SetShowCloseButton(false);
-  SetModalType(ui::MODAL_TYPE_WINDOW);
+  SetModalType(ui::MODAL_TYPE_SYSTEM);
   set_fixed_width(views::LayoutProvider::Get()->GetDistanceMetric(
       views::DISTANCE_MODAL_DIALOG_PREFERRED_WIDTH));
 }