IDL compiler: Remove dup assignment

17 => 16

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/137503003

git-svn-id: svn://svn.chromium.org/blink/trunk@166447 bbb929c8-8fbe-4397-9dbb-9b2b20218538