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Remove weak pointers from CastSocket by explicitly tracking and resetting callbacks created inside the class.

Ensure all sockets are closed and callbacks reset in all relevant code paths: Close(), CloseWithError(), and the dtor.

Review URL: https://codereview.chromium.org/417403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287477 0039d316-1c4b-4281-b951-d872f2087c98