Devices reporting exactly 512MB are currently not categorized as low-end
thanks to a < instead of <=.  Because of reserved amounts, physical 512MB
devices are reported as lower, so there isn't a known case where this makes
any difference, but this line of code has caused confusion a few times, so change
it for clarity's sake.

BUG=607311

Review-Url: https://codereview.chromium.org/1925813002
Cr-Commit-Position: refs/heads/master@{#390446}