Changed Message::content() to pass $this to MessageContent.

MessageContent can take either a Message or a message key in
the constructor. Since Message::content() is an instance method,
it makes sense to pass the instance itself rather than just
the key. This fixes cases where RawMessage::content() is called,
since passing RawMessage's key to MessageContent will not work
properly.

Change-Id: Ic3b935e5ac997df07eea40b438445ebd5024e306
Merge "Changed Message::content() to pass $this to MessageContent."