This relands parts of "[heap] Allow a minimum semi-space size of 512K."
excluding the actual semi-space size change.

This partially reverts commit f341bb0f

> Original commit message:
> Revert "[heap] Allow a minimum semi-space size of 512K."
> This reverts commit 0d2ed6c3.
> The CL introduced perf regressions: crbug.com/735649.
> We are going to reland the CL in an isolated V8 roll to ensure
> that perf regressions are attributed correctly.

> 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

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I5ed66b72104aa877d67fcd20bdadc807ea1551c3
Reviewed-on: https://chromium-review.googlesource.com/575065
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46762}