Oilpan: add explicit Persistent<T>(T&) and Member<T>(T&) constructors.

Synchronize with RefPtr<T>(T&)'s constructor offering and provide same,
explicit constructors taking a reference.

Make use of it in PostMessageTimer and WorkerThreadableWebSocketChannel.

R=haraken@chromium.org,ager@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/221823002

git-svn-id: svn://svn.chromium.org/blink/trunk@170792 bbb929c8-8fbe-4397-9dbb-9b2b20218538