Skip write barriers in the fast case when setting up local context.

Skip write barriers in the fast case when setting up local context.

The FastNewContextStub always allocates in new space, so we don't
need to update the write barrier when copying the parameters to
the newly allocated context.

R=hpayer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=21481

[Benedikt Meurer, 2014-05-24 12:24:31]

PTAL

[Hannes Payer (out of office), 2014-05-26 08:12:20]

lgtm

[Benedikt Meurer, 2014-05-26 08:13:19]

Committed patchset #1 manually as r21481 (presubmit successful).