This is a partial reland of "Allow a minimum semi-space size of 512K."
with a heuristic to keep 1MB initial semi-space size for high
memory devices.

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,chromium:716032

Change-Id: I5ec851f2380c40dbc34b4576a668f63ae38ac898
Reviewed-on: https://chromium-review.googlesource.com/594387Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Hablich <hablich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47065}