In contrast to other internal fields (data, source, and flags), last_index is
an in-object property. But we can still use the standard accessor macros to
access it.

Bug: 
Change-Id: If77f2bb01c6ddccebdde09d7a316c2ddaaf9b277
Reviewed-on: https://chromium-review.googlesource.com/577549Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46779}