Remove an unused typedef. No behavior change.

Found by a prototype of -Wunused-local-typedef for clang.

BUG=none

Review URL: https://codereview.chromium.org/427473002

git-svn-id: svn://svn.chromium.org/blink/trunk@179005 bbb929c8-8fbe-4397-9dbb-9b2b20218538