Always add_includes_for_types for attributes

Per earlier request, this simplifies the CG by always adding includes
for types, rather than burying it in the getter/setter code.

Only changes to generated code are 5 additional includes:
* Add "bindings/v8/ScriptValue.h" to 4 .cpp files that use
[ConstructorTemplate=Event] interface and [Custom] attribute
...b/c these use |any| for the attribute type, which is hard-coded in the
[ConstructorTemplate=Event] code

* Add "V8Clipboard.h" to V8Event.cpp, which is correct,
  and in fact included in the custom bindings in V8EventCustom.cpp

This turned up a bug:
* Some [Custom] attributes had meaningless types, like |custom| or |Array|
Trying to add includes for these fails.

I've changed the IDL files:
I've replaced these meaningless types with |object|, which
is the generic type.
I considered using |void|, but that's confusing, b/c it's empty, not generic.
Also could use |any| (as above), but that generates ScriptValue.h includes.


Also some related cleanup:
* This significantly flatten v8_attributes.py,
moving lots of logic into |generate_attribute|, so
|generate_getter| is simpler (only the code it actually needs) and
|generate_setter| can be eliminated!

* Removes a bit of duplicated code in Python.

* Removed a duplicate variable in the Perl code.

Review URL: https://codereview.chromium.org/61793018

git-svn-id: svn://svn.chromium.org/blink/trunk@162005 bbb929c8-8fbe-4397-9dbb-9b2b20218538