IDL compiler: [Reflect] for setters

Rather big (as expected), but primarily refactoring of existing getter code.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/33543005

git-svn-id: svn://svn.chromium.org/blink/trunk@160131 bbb929c8-8fbe-4397-9dbb-9b2b20218538