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/210992Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>