Merged r14732, r14746, r14757 into 3.18 branch.

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

Fix unexpected elements transition in JSON.parse

Fix bogus deopt in BuildEmitDeepCopy for holey arrays.

BUG=241344,chromium:242924
R=verwaest@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15941005

git-svn-id: https://v8.googlecode.com/svn/branches/3.18@14799 ce2b1a6d-e550-0410-aec6-3dcde31c8c00