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`SlowPathAlloc` has 3 tactics for getting new memory. Of these, only the 1st
(calling `PartitionDirectMap`) seems to most obviously get a fresh, zeroed page
from the OS. I think it is safe to optimize in that branch. It might be safe on
some platforms to optimize in the other 2 branches, but it's less obviously
safe, and we've had trouble on at least macOS. This CL enables the 1st branch
but turns off the optimization in the 2nd 2 branches.

Bug: 890752
Cq-Include-Trybots: luci.chromium.try:mac_optional_gpu_tests_rel
Change-Id: Ibd9ff599ba78aeab47c2369c472af71a4f71d600
Reviewed-on: https://chromium-review.googlesource.com/1256211Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596297}