Merged r14171, r14175 into trunk branch.

Move context retrieval method around. Use delegation for implementation.

Fix slow path of JSON.stringifier when GC strikes.

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/13839008

git-svn-id: https://v8.googlecode.com/svn/trunk@14182 ce2b1a6d-e550-0410-aec6-3dcde31c8c00