[deprecate-unload] Switch from onunload to onpagehide

This CL changes unload to pagehide handler so it would be
triggered as expected even after unload is deprecated in Chrome.
Pagehide can also be used to detect document destruction. In other browsers,
pagehide is also triggered for the case that is tested by the test
, and is more reliable than unload.

Change-Id: Ic6059bc59890d69815ab84a9d0b0585dc8a7d08e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5108724
Commit-Queue: Nonoka Muraki <murakinonoka@chromium.org>
Reviewed-by: Ming-Ying Chung <mych@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1236812}