Calculate the aligned size of strings when allocating from generated code using kObjectAlignmentMask

Add native code assert to make sure allocation from generated code keeps alignment.
Review URL: http://codereview.chromium.org/462025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3415 ce2b1a6d-e550-0410-aec6-3dcde31c8c00