Pass a callback into the install dialog which does the install.

When 'Install' is clicked, the callback will be run. The caller must call SetInstallSuccess so the dialog knows when the install has finished.

This change also hooks this up to the omnibox install behind a flag.

Bug: 1488697, b/307146382
Change-Id: I7c7a7016cd86cec9184f74508badad1b7de1f877
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5028373
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Louise Brett <loubrett@google.com>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1229767}