IDL compiler: merge getter and setter PerWorldBindings loops

A minor simplification, making the code a bit clearer and more consistent.
Just rearranges generation order, otherwise no changes.

BUG=239771

Review URL: https://codereview.chromium.org/37093002

git-svn-id: svn://svn.chromium.org/blink/trunk@160335 bbb929c8-8fbe-4397-9dbb-9b2b20218538