ARM64 has 39-bits of address space.

We can do better than than current behaviour (which falls through to the 32-bit
path).

NOTRY=true
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/290643004

git-svn-id: svn://svn.chromium.org/blink/trunk@174308 bbb929c8-8fbe-4397-9dbb-9b2b20218538