- Decrease regular heap object size to 300k, keeping the same ration (60%)
  between this limit and page size.

In a follow up, we can now get rid of the new space border page while
keeping the 1M minimum new space size.

Some results (v8.infinite_scroll; 3 runs):
- evacuate.avg: +15.3% (1.4->1.2)
- evacuate.max: +24.4% (2.4->1.8)

BUG=chromium:581412
LOG=N
R=hpayer@chromium.org, ulan@chromium.org, yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2013713003
Cr-Commit-Position: refs/heads/master@{#38533}