This change removes the large contiguous backing store from the young generation
and replaces it regular pages.

We keep a pool of pages that are committed/uncommitted to avoid creating virtual
memory maps during growing and shrinking.

BUG=chromium:581412
LOG=N

Review URL: https://codereview.chromium.org/1853783002

Cr-Commit-Position: refs/heads/master@{#35261}