[Cast2Class] Close dialog on deactivation

Close the Cast2Class discovery dialog when it loses focus. This is
needed since casting relies on the browser context, and we cannot make
the dialog system modal. Additionally, adds 'delete this' to the
WebDialogDelegate's OnDialogClose() method to ensure the delegate is
deleted when closed, to prevent memory leak.

PLEASE NOTE the 'window.onblur' implementation is a temporary fix while
we investigate a long term solution.

Demonstration of dialog closing on deactivation (un-focus)
https://screencast.googleplex.com/cast/NjY4MjQ5MDg5NTg1OTcxMnwxZGJkZmEzMS0zZA

Bug: b/213497443
Change-Id: Ia2ab9f1733497819f45a72f8968e87d54aa52de3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3372607
Reviewed-by: Brian Malcolm <bmalcolm@chromium.org>
Commit-Queue: Benjamin Zielinski <bzielinski@google.com>
Cr-Commit-Position: refs/heads/main@{#956669}