Add support for a target new space size

When this flag is set, we will commit additional pages up until the
target size before doing a scavenge.

I made sure that all tests pass when a target size of 16MB is set.

BUG=v8:3626
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/652543007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24790 ce2b1a6d-e550-0410-aec6-3dcde31c8c00