As part of that change, make ToNumber return in the accumulator.

Bug: v8:6791
Change-Id: I8ce0f4fbc7ad8ee7fb4a32a8a499394395010750
Reviewed-on: https://chromium-review.googlesource.com/658082Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47976}