Fix build warning

We're only supposed to list this IDL file once in the GYPI.

TBR=dslomov@chromium.org

Review URL: https://codereview.chromium.org/22378005

git-svn-id: svn://svn.chromium.org/blink/trunk@155701 bbb929c8-8fbe-4397-9dbb-9b2b20218538