Fix bindings when using [CachedAttribute] on Array.

This make sure we call the proper cpp_value_to_v8_value instead of
doing {cpp_value}.v8value() which doesn't work for all types.

BUG=374146

Review URL: https://codereview.chromium.org/282263007

git-svn-id: svn://svn.chromium.org/blink/trunk@174161 bbb929c8-8fbe-4397-9dbb-9b2b20218538