Prevent PageAllocator from fragmenting address space on small Windows systems.

Peform a runtime check in the 32-bit windows code only to see if the
address space is highly constrained by the OS. If so, we give up some
randomness in exchange for memory utilization efficiency.

BUG=394591

Review URL: https://codereview.chromium.org/408483004

git-svn-id: svn://svn.chromium.org/blink/trunk@178586 bbb929c8-8fbe-4397-9dbb-9b2b20218538