[Autofill Assistant] Remove unnecessary round() from ElementPositionGetter.

Before this change, ElementPositionGetter would round each coordinate it
got before computing the center, then rounding it. With this change,
only the center is rounded. This was puzzling.

This change is expected to have no effect, as all coordinates should be
integer values to begin with. It's just a cleanup.

Change-Id: Ib0ac75f6e08d825f1521e32325c87672eb9f9d20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521734


Reviewed-by: Clemens Arbesser <arbesser@google.com>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824839}