Give explicit value to EnabledAtRuntime (IDL extended attribute)

Several IDL extended attributes take an optional value, falling back
to various defaults if the value is absent.
In practice, other than Reflect, the value is almost always used.
The fallback makes the code generator substantially more complicated,
and the behavior opaque, so instead we're making the value required (if possible).

Implementing this is just filling in the value explicitly in the few
places where it's not given, then removing the fallback from the code
generator and updating tests.

I'm starting with EnabledAtRuntime, which this CL fixes.

Review URL: https://chromiumcodereview.appspot.com/23667002

git-svn-id: svn://svn.chromium.org/blink/trunk@156831 bbb929c8-8fbe-4397-9dbb-9b2b20218538