Merged r14732, r14734 into 3.17 branch.

Zap map value in the stub-cache on clear. The empty_string is a valid key for keyed loads and stores.

Add regression test for fix from r14732.

R=jkummerow@chromium.org
BUG=chromium:242502

Review URL: https://codereview.chromium.org/16084007

git-svn-id: https://v8.googlecode.com/svn/branches/3.17@14840 ce2b1a6d-e550-0410-aec6-3dcde31c8c00