Avoid looping over role allocations during headroom computation.
Avoid looping over role allocations during headroom computation.

Now that the sorter exposes the allocation information for the
root of the tree, we can simply use that instead of looping over
the top level roles.

Overall against master this branch shows negligeable differences
in both the *BENCHMARK_HierarchicalAllocator_*.LargeAndSmallQuota*
and *BENCHMARK_HierarchicalAllocator_*NonQuotaVsQuota/4 benchmarks.

Review: https://reviews.apache.org/r/70382

reneploetz/mesos,reneploetz/mesos,chhsia0/mesos,jpeach/mesos,Gilbert88/mesos,jpeach/mesos,abudnik/mesos,gsantovena/mesos,abudnik/mesos,Gilbert88/mesos,chhsia0/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,jpeach/mesos,gsantovena/mesos,kaysoky/mesos,Gilbert88/mesos,chhsia0/mesos,chhsia0/mesos,gsantovena/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,abudnik/mesos,Gilbert88/mesos,reneploetz/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos