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In Harmony, the distance between these two elements is supposed
to be 3/4 of a layout unit instead of one layout unit, so:

1) Add DISTANCE_BUTTON_IMAGE_LABEL_PADDING to DistanceMetrics
2) Give it the same default value as DISTANCE_RELATED_CONTROL_HORIZONTAL
3) Add a Harmony value for the new DistanceMetric

Bug: 794480
Change-Id: Ib7106612dd2c923386cdbf30219ff37180acdedd
Reviewed-on: https://chromium-review.googlesource.com/848533Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526963}