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Previously, task nodes were all fed the same priority. While certain
heap implementations ensured a consistent prioritization given the same
task priority and input ordering, this is not guaranteed by the spec.
In particular, Android's libc++ implementation appears to differ in this
respect, resulting in tasks from different sets being processed in an
order different from node creation order.

Avoid this by including the task set type in the node priority. All
conforming heap implementations should now yield the same ordering
with respect to task set type.

BUG=427718

Review URL: https://codereview.chromium.org/875573006

Cr-Commit-Position: refs/heads/master@{#313972}