Revert of "[heap] Switch to 500k pages" (patchset #11 id:220001 of https://codereview.chromium.org/2232653003/ )

Reason for revert:
Breaks benchmark with --turbo on avx2

https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20-%20avx2/builds/9895

Original issue's description:
> Reland of "[heap] Switch to 500k pages"
>
> Decrease regular heap object size to 400k. In a follow up, we can now get rid of
> the new space border page while keeping the 1M minimum new space size.
>
> BUG=chromium:636331
>
> This reverts commit 555c9619.
>
> Committed: https://crrev.com/20e2ea80e169e85c5b8231adc02901fb6c989609
> Cr-Commit-Position: refs/heads/master@{#38608}

TBR=hpayer@chromium.org,yangguo@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:636331

Review-Url: https://codereview.chromium.org/2239323002
Cr-Commit-Position: refs/heads/master@{#38613}