Auto merge of #3491 - jrmuizel:lp-clear, r=nical
Auto merge of #3491 - jrmuizel:lp-clear, r=nical

Clear blob_requests on LowPrioritySceneBuilder.

PR #3398 made it so we weren't clearing txn.blob_requests
on the low priority scene builder. I believe this change was
unintentional. Clearing blob_requests cuts the amount of
blob rasterization that we do in half on hixie-004.xml and
should fix the talos regression that we were seeing.

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