3.20 branch: Fix HObjectAccess for loads from migrating prototypes

BUG=chromium:305309
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/37253003

git-svn-id: https://v8.googlecode.com/svn/branches/3.20@17655 ce2b1a6d-e550-0410-aec6-3dcde31c8c00