Auto merge of #2490 - servo:kvark-content-rect, r=staktrace
Auto merge of #2490 - servo:kvark-content-rect, r=staktrace

Ensure non-zero picture task size

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1442718

~~Try push: https://treeherder.mozilla.org/#/jobs?repo=try&revision=dc0d35306d006a09ddb0cf7db453a97fa9bf8dca~~

Another try push: https://treeherder.mozilla.org/#/jobs?repo=try&revision=11b5d65d0e682c277992c1cea5d4048227184118

r? @staktrace

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