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The SelfUninstall and SelfUninstallNoPermissions test are flaky when
run by a SW-based extension. This CL moves them to the base class,
so they are only run as legacy extensions.

Bug: 1132581
Change-Id: Iec1fce201f58d833b7d9b124e658d0ae0219db1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446836Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814464}