IDL: [CustomEnumerateProperty] => [Custom=PropertyEnumerator]

Merge another [Custom*] into [Custom=*].
Only one use, CSSStyleDeclaration.idl (named property getter).

Also have [Custom=PropertyGetter] when need to combine with
[Custom=PropertyEnumerator] (defaults to just making the getter custom).

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/146873003

git-svn-id: svn://svn.chromium.org/blink/trunk@166113 bbb929c8-8fbe-4397-9dbb-9b2b20218538