Add browser_tests to the experimental MSan bot.

Eventually we plan to move them to a separate runner. However, that requires an
extra machine to set up a dedicated builder, which we don't currently have.

BUG=178409
R=glider@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/333323002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@277415 4ff67af0-8c30-449e-8e8b-ad334ec8d88c