IDL compiler: remove [Unserializable]

Only one case, instead special-case the interface:
(per discussion below, unlikely to be used for other interfaces)

One minor change to generated code: add header to V8ErrorEvent.cpp
to simplify Python (avoid a special case).

core/events/ErrorEvent.idl
[Custom=Getter, InitializedByEventConstructor, Unserializable] readonly attribute any error;

This also simplifies the |any| special-casing in [EventConstructor] by using
an auxiliary list, as suggested by haraken@.

For reference:
[Unserializable] was added by Mike West (CC'ed) in:
Add 'error' parameter to 'window.onerror' handlers.
https://chromiumcodereview.appspot.com/20351002

Updated docs (removed):
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-Undocumented-FIXME

BUG=239771

Review URL: https://codereview.chromium.org/110983003

git-svn-id: svn://svn.chromium.org/blink/trunk@163687 bbb929c8-8fbe-4397-9dbb-9b2b20218538