This ensures that 2GB devices get 512K initial semi-space.

Bug: chromium:735649
TBR: mlippautz@chromium.org
Change-Id: I864c9dfa7cd3ea9040fa56d81d6bf1187c1cf776
Reviewed-on: https://chromium-review.googlesource.com/608129Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47246}