RenderDocument: Fix BeforeUnload tests.

The BeforeUnload* tests call a helper function that turns off the unload
hang monitors in the active RFHs at the start of the test. Without it,
when the beforeunload dialog appears, it will prevent the browser from
quitting, which will cause the test to hang.

With RenderDocument enabled, the navigation results in a frame swap, and
the new RFH that's created doesn't have its hang monitor disabled,
resulting in the test timing out during the post-test cleanup process.
This CL fixes these tests by disabling the hang monitor again at the end
of the test for all active RFHs, ensuring that any RFHs created during
the test also have their unload hang monitors turned off before the
cleanup stage.

Change-Id: I8afa15cd7b518b2a6ad797beb5a583e7c3a79202
Bug: 40615943
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6871815
Commit-Queue: Liam Brady <lbrady@google.com>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1505410}