bindings: Fix header inclusions about IDL observable array types

The Blink-V8 bindings code generator didn't include the header
for an IDL observable array type (i.e.
`#include ".../v8_observable_array_xxxx.h"` was missing in the
generated bindings code).

This patch makes the bindings code generator produce the header
inclusion when an attribute type is an IDL observable array type.

This patch also demonstrates the fix by removing the header
inclusion of "v8_observable_array_css_style_sheet.h" from
t/b/r/code/dom/tree_scope.h.

Change-Id: I1802cce6060291e42150ad1daa4d228a722bd5ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965399
Reviewed-by: David Sanders <dsanders11@ucsbalum.com>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518277}