Avoid HEAP macro in spaces.cc

This was another top cause for TLS access.

BUG=v8:2487

Review URL: https://codereview.chromium.org/12340101

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13755 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Added Isolate parameter to CodeStub::GetCode().

According to gprof, this was the #1 cause for TLS access during an Octane run.

BUG=v8:2487

Review URL: https://codereview.chromium.org/12317141

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13756 ce2b1a6d-e550-0410-aec6-3dcde31c8c00