IDL: Enumerations support in dictionaries

Enumerations are essentially the same as Strings, but those values
are restricted to specific values. When a value is invalid, we should
throw an exception as the spec requires[1].

[1] http://heycam.github.io/webidl/#es-enumeration

BUG=321462

Review URL: https://codereview.chromium.org/561633003

git-svn-id: svn://svn.chromium.org/blink/trunk@181729 bbb929c8-8fbe-4397-9dbb-9b2b20218538