FILE PHOTO: The United Nations headquarters building is pictured though a window with the UN logo in the foreground in the Manhattan borough of New York August 15, 2014. REUTERS/Carlo Allegri Reuters By Michelle Nichols UNITED NATIONS (Reuters) - The United Nations General Assembly on Thursday agreed on a definition of denial of the Holocaust, when the Nazis killed 6 million Jews during World War Two, and urged social media companies "to take active measures" to combat antisemitism. "The General Assembly is sending a strong and unambiguous message against the denial or the distortion of these historical facts," said Germany's...