blink: updates http/tests/history/back-during-beforeunload.html

I'm in the process of making html parsing happen a bit earlier than
previously (specifically it removes a posttask). This has lead to
some flake surfacing in tests.

This particular test is installing a before-unload handler. The
before unload handler calls history.back() and then notifyDone().
It then sets a timeout that changes the location. The expectations
are such that the location does not change.

notifyDone() may not immediately stop the test. In particular, if
a frame is loading it will not stop. With the post task removed it
the timeout can now fire (largely in debug builds) before the
document has finished loading. This means notifyDone() doesn't stop
and the expectations end up wrong. The fix is to move the
timeout to onload, so that we know the document has finished loading
and notifyDone() will stop the test.

Bug: 1364695

Change-Id: Ifb11b0482599afb997dafee36ba859fa427a7d48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3963632
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1060760}