Silicon Valley firms were prepared this week to quickly block video of an Islamic State militant beheading an American journalist after a previous video by the same group showing the death of James Foley ricocheted through social networks in what was seen by some as a propaganda coup for the extremists. The video Tuesday showing the beheading of American journalist Steven Sotloff was first uploaded onto a different website and quickly deleted when copied onto YouTube, slowing the spread of posts linking to it, said a Silicon Valley insider, speaking on condition of anonymity because of the sensitivity of the...