Support missing and invalid defaults for reflected attributes.

Some enumerated attributes have a designated state if the underlying
attribute value is either missing or in an invalid state. That state
should be observable for reflected IDL attributes on getting, hence
add support for controlling what values to use for these attribute
states:

  [ReflectMissing=missingValue]
  [ReflectInvalid=invalidValue]

The attributes only have an interpretation for limited DOMString
attributes that use [ReflectOnly=<list>]. The missing/invalid value
defaults must appear in the ReflectOnly list.

R=
BUG=331694

Review URL: https://codereview.chromium.org/98033003

git-svn-id: svn://svn.chromium.org/blink/trunk@164579 bbb929c8-8fbe-4397-9dbb-9b2b20218538