Also handle elements in *RealNamed* api methods

Also handle elements in *RealNamed* api methods

Apparently the *RealNamed* API methods only have named variants, but were always used by the embedder to find elements as well. We'd never find them though, since we wouldn't even look
there.

This CL ensures we check whether the name is actually an array index.

I guess for all named API functions we should assume they are used similar to o["name"] where name could also be a number... At least we should make it uniform between embedder and V8. This matches us up with their expectations for now...

BUG=v8:4137
LOG=n

Review URL: https://codereview.chromium.org/1177383004

Cr-Commit-Position: refs/heads/master@{#29008}