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With this CL you can now iterate across elements both forward and
backward (for rendering and hit testing, respectively).

Bug: None
Change-Id: Iadc1f43353ac5ed8cef550103bac223129802035
Reviewed-on: https://chromium-review.googlesource.com/672605
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502926}