This patch changes the semi-space size to 512K.

> 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

Change-Id: Iabc377cba2911b28d51b98bb5b85134d4e893632
Reviewed-on: https://chromium-review.googlesource.com/575066Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46763}