Methods in the runtime that enumerate over properties should never deal with private symbols. Most commonly such methods only loop over enumerable properties. This fix avoids accidentally handling private symbols in methods that only deal with enumerable properties. Methods that need to look at non-enumerable properties as well still have to manually filter private symbols (e.g., the KeyAccumulator).

BUG=chromium:664411

Review-Url: https://codereview.chromium.org/2499593002
Cr-Commit-Position: refs/heads/master@{#40932}