[Web Apps Infra] Position app install dialog at top of parent window

Before and after screenshots attached to issue (b/312599130)

Bug: b/312599130
Change-Id: Idf9c339e2ef7068001c90a768acbb543acb761d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5167727


Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280481}