Performance regressed for this with the I+TF switch. This speeds up
the simple case by using optimizations in the elements accessor.

Bug: chromium:700835
Change-Id: Iaba30951b93daefa0fb32acd6656ac705cdc73ed
Reviewed-on: https://chromium-review.googlesource.com/483341
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44913}