Properly skip tests if no elevation.

Some tests from CookieEncryptionProviderBrowserTest require elevation
but don't handle absence of elevation properly and fail instead of being
skipped.
Skip all parts of test if test or its pretest require elevation and
don't check metrics for skipped tests in TearDown.

Change-Id: Ibf2d6b1dcf649aea4278f5b623ecb3b84ee92712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5569459
Auto-Submit: Alexander Kalugin <akalugin@yandex-team.ru>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1308151}