As violence spread in the Arab world over a video on YouTube ridiculing the Prophet Muhammad, Google, the owner of YouTube, blocked access to it in two of the countries in turmoil, Egypt and Libya, but did not remove the video from its Web site. “Google is the world’s gatekeeper for information, so if Google wants to define the First Amendment to exclude this sort of material then there’s not a lot the rest of the world can do about it,” said Peter Spiro, a constitutional and international law professor at Temple.