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Previously we used the center of the bounding box, but this can cause problems if the center of the bounding box is outside the element.

This CL also changes the behavior of the GetElementLocationOnceScrolledIntoView command in the same way.

BUG=

Review URL: https://codereview.chromium.org/848483002

Cr-Commit-Position: refs/heads/master@{#311423}