3.21 branch: Fix HObjectAccess for loads from migrating prototypes

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

Review URL: https://codereview.chromium.org/37323002

git-svn-id: https://v8.googlecode.com/svn/branches/3.21@17406 ce2b1a6d-e550-0410-aec6-3dcde31c8c00