Document that tests should avoid using bare SWPs.

Instead, SequencedWorkerPools should be wrapped in a SequencedWorkerPoolOwner to
avoid leak reports.

BUG=273800
R=ajwong@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/102753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238451 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238451