Fix gcstress test failure

Map collection complicates a test that wants to assert on code opt/deopt
because of prototype-chain changes. It can happen that a gc occurs
in the stack guard at the start of optimized function foo that deopts
function foo because of a map being collected and deoptimizing it's
dependent code.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/159653002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19258 ce2b1a6d-e550-0410-aec6-3dcde31c8c00