We must print "[object Array]" for proxies that satisfy Array.isArray.

Cosmetic change on the side: move ObjectProtoToString from JSObject to Object
since it deals with arbitrary objects.

R=adamk@chromium.org, verwaest@chromium.org
BUG=v8:1543
LOG=n

Review URL: https://codereview.chromium.org/1526023002

Cr-Commit-Position: refs/heads/master@{#32902}