Re-land: 178586 Avoid fragmenting address space on small windows systems.
Re-land: 178586 Avoid fragmenting address space on small windows systems.

The concern about IsWow64Process() is not founded per jschuh@ (but the effect of
changing the allocator behaviour may be). We now have a clean baseline to test
against.

R=cevans@chromium.org
BUG=394591

Review URL: https://codereview.chromium.org/717263005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@185559 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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