This matches the approach used for @@isConcatSpreadable, and seems to
match what Mozilla is planning to do in Firefox.

Given that there's already little compatibility around cross-origin toString
results, there seems to be little hazard in making this change even before
spec language hits the HTML spec.

BUG=v8:3502, v8:4289, chromium:532469
LOG=n

Review URL: https://codereview.chromium.org/1432543002

Cr-Commit-Position: refs/heads/master@{#31755}