Fix crash in LoginHandlerViews.

The destructor of WebContents can trigger
WebContentsObserver::DidFinishNavigation. This in turn can trigger the
LoginTabHelper process for showing a Loginhandler. At this point, the
WebContentsModalDialogManager has already been destroyed so the http
auth dialog can no longer be shown.

Change-Id: I5577a1d2cbeae034ad2217f1de546f459eb4885f
Bug: 328462789, 328462789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5355654


Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270490}