Auto merge of #6841 - pcwalton:squash-animation-ticks, r=metajack
Auto merge of #6841 - pcwalton:squash-animation-ticks, r=metajack

script: Squash duplicate animation tick events in the queue.

This prevents them from piling up endlessly if the JS
`requestAnimationFrame()` callback takes a long time to complete.

r? @jdm

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