Revert of "[heap] Switch to 500k pages" (patchset #1 id:1 of https://codereview.chromium.org/2278653003/ )

Reason for revert:
Tanks pretty much alle metrics across the board. Probably LO space limit too low but needs investigation.

Original issue's description:
> [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.
>
> This reverts commit 1617043c.
>
> BUG=chromium:636331
>
> Committed: https://crrev.com/2101e691caeef656eb91f1c98620b3955d337c83
> Cr-Commit-Position: refs/heads/master@{#38916}

TBR=ulan@chromium.org,verwaest@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:636331
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2289493002
Cr-Commit-Position: refs/heads/master@{#38960}