Don't align size on allocation granularity for unaligned ReserveRegion calls.

Also add additional ASSERTs to help tracking the flaky
test-alloc/CodeRange in Windows.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23542027

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16648 ce2b1a6d-e550-0410-aec6-3dcde31c8c00