MSan: mark any memory allocated from the JS heap as uninitialized.

BUG=chromium:403409,chromium:178409
R=jkummerow@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/480763003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23268 ce2b1a6d-e550-0410-aec6-3dcde31c8c00