(Fixes date in dateline, no change to content of story) By Howard Schneider and Ann Saphir WASHINGTON (Reuters) – By some early estimates, the U.S. economy, as measured by gross domestic product, may have shrunk in the three months from April through June. Add that to the decline from January through March, and that would be a contraction for two quarters in a row. By an often-cited rule of thumb, that means the world’s largest economy is in recession. But deciding when a recession has begun or predicting when one might occur is not straightforward. The “two quarters” definition is handy for a…