Introduce a flag to change the new space growth factor

A useful value would be 4, so we get 1, 4, 16MB (instead of the default
value 2 which leads to 1, 2, 4, 8, 16)

BUG=none
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/753513002

Cr-Commit-Position: refs/heads/master@{#25462}