Oilpan: remove unnecessary WeakMember friend class decl.
Oilpan: remove unnecessary WeakMember friend class decl.

R=kouhei
BUG=420515

Review URL: https://codereview.chromium.org/792233006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187381 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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