Add an --msan flag to run-tests.py.

Currently a no-op and added only for consistency with ASan/TSan.

BUG=chromium:425187
R=machenbach@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/667903004

Patch from Sergey Matveev <earthdok@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24805 ce2b1a6d-e550-0410-aec6-3dcde31c8c00