Fix use-after-destroy in DlpContentManagerAshTest

The unit test does not have the full stack of objects normally used to
manage DlpContentManager's list of WebContents. Manually instantiate the
associated TabHelper to ensure that WebContents are removed from
DlpContentManager::confidential_web_contents_ on destruction.

To go along with this, the test fixture also now consistently sets up
the reporting manager and the rules manager. There are no tests that
require these to *not* be set up; in contrast, DlpContentTabHelper
*does* require that the rules manager be set up as a bare minimum.

Bug: 40222690
Change-Id: I9950d0131cc7d1440bc998a9ba6795746e334522
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5798036
Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1344666}