Remove hard-coded assumption of large object size threshold.

This test fails on PPC in version 4.7 where the threshold is derived
directly from the allocator's pagesize.

R=hpayer@chromium.org, mstarzinger@chromium.org, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1440723002

Cr-Commit-Position: refs/heads/master@{#31963}