This sets the minimum semi-space size to 512K, but does not
change the initial semi-space size.

This reverts commit 774a4c5e.

Original commit message:
> [heap] Allow a minimum semi-space size of 512K.
> This CL also reduces the minimum semi-space size to 512K.
> BUG=chromium:716032
BUG=chromium:735649
TBR=mlippautz@chromium.org

Change-Id: I1f5dd05b0851ba2b438bedcc023a5cf5f9242a81
Reviewed-on: https://chromium-review.googlesource.com/594107Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47016}