Document CrossThreadWeakPersistent.
Document CrossThreadWeakPersistent.

This helps people who follow a link directly to the class without reading its
file top-down.

Review URL: https://codereview.chromium.org/1305933006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@201655 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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