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This cl converts middleClickAutoscroll.js and the middle click autoscroll
layout tests that are using it to use gpu benchmarking.

Bug: 875945
Change-Id: Ic5acb4367dd406de3e5897063bfe6b1b44e7daa6
Reviewed-on: https://chromium-review.googlesource.com/1251901Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595596}