[mparch] Handle a dialog in Page.disable() only if the page is primary

This CL handles a dialog after checking if a page is primary in
PageHandler::Disable() as a dialog is not opened in non primary pages.

It also introduces a test that Page.disable() works based on a page.

Bug: 1324905
Change-Id: Iedee41a226afda9563305da47213b99712e65a9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3692621


Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1012857}