v8binding: Support mixture of [SecureContext] and inherited interface member

Inherited IDL interface member shouldn't be affected by
[SecureContext] applied on an inheriting IDL interface.  This patch
fixes it.

Example

    interface Base {
      [Unforgeable] attribute T attr;
    };

    [SecureContext]
    interface Derived : Base {
      attribute T attr2;
    };

When an instance of |Derived| is created in non-secure context,
the instance does not have |attr2| due to [SecureContext], however,
the instance must have |attr| because Base's attr is not specified
with [SecureContext] although |attr| is inherited to Derived.

Bug: 933731
Change-Id: I1ffb8acb0ead8d6e378c6f51a131b5779ddd293c
Reviewed-on: https://chromium-review.googlesource.com/c/1478668
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634122}
4 files changed