With the recent spec change removing the [[Enumerate]] internal method, we now
have to walk the complete prototype chain. This implies that we call the
[[GetPrototypeOf]] trap on proxies.

As a secondary change we now trigger the [[GetOwnProperty]] trap for the for-in
filter step to see whether the properties are still enumerable. Before we did this
in the key-accumulation phase. This way we slightly reduce the number of traps
invoked. Whilst this is not ideal, it comes closer to the Spec's example
implementation.

BUG=v8:1543, v8:4768
LOG=n

Review URL: https://codereview.chromium.org/1748923003

Cr-Commit-Position: refs/heads/master@{#35017}