Make the HugeSetDataTest more robust.
Make the HugeSetDataTest more robust.

By starting from a near-maximum size, and trying until we reach a
a valid allocatable size, we can guarantee the test can run.
Previously the allocation could fail under some systems and
succeded in others, leading to flakiness.

BUG=angle:716

Change-Id: If1690349f7028c4e6a88f20649fb255ea2dd2587
Reviewed-on: https://chromium-review.googlesource.com/210992
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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