Initialize members in TypeConversions test-only interface

Layout tests under MSAN FTW!

BUG=416415
R=earthdok@chromium.org
TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/593763002

git-svn-id: svn://svn.chromium.org/blink/trunk@182440 bbb929c8-8fbe-4397-9dbb-9b2b20218538