IDL compiler: remove [MasqueradesAsUndefined]

This removes the [MasqueradesAsUndefined] extended attribute, instead
including a special case for the one interface that uses it.
(HTMLAllCollection, for document.all)

Response to request of haraken@:
https://codereview.chromium.org/103753002/#msg2

* No changes to generated code.
* Removed from IDLExtendedAttributes.txt
* Docs updated to remove mention.
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes

BUG=239771

Review URL: https://codereview.chromium.org/103813002

git-svn-id: svn://svn.chromium.org/blink/trunk@163163 bbb929c8-8fbe-4397-9dbb-9b2b20218538