Revert 114494 - Remove BindStateHolder and have Bind() return a Callback<> object directly.

Revert 114494 - Remove BindStateHolder and have Bind() return a Callback<> object directly.

This removes some complexity and also fixes a bug where if you call Bind() with the result of Bind(), the resulting Callback would only be valid during the first call.  Ouch.

BUG=none
TEST=new unittests


Review URL: http://codereview.chromium.org/8738001

TBR=ajwong@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=114495