Only assign environments when they are actually needed. (x64 only)

Only assign environments when they are actually needed. (x64 only)

Twin of https://code.google.com/p/v8/source/detail?r=20231.

R=yangguo@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=20271

[Yang, 2014-03-26 10:02:54]

On 2014/03/25 13:52:09, Sven Panne wrote:

lgtm.

[Sven Panne, 2014-03-26 10:35:43]

Committed patchset #1 manually as r20271 (presubmit successful).