When possible (non-Proxy receiver, expecting only String-names), walk the instance_descriptors() array rather than performing [[OwnPropertyKeys]]. If the map changes during a call to an accessor property, resort to a slower property lookup.

For now, the fast path is not taken if the object contains any element keys.

Offers a measurable improvement over the existing version, in select situations.

BUG=v8:4663
LOG=N
R=cbruni@chromium.org, verwaest@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/1751643003

Cr-Commit-Position: refs/heads/master@{#34567}