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The spacing around the footer hasn't been consistent across all sheets.
This CL generalizes the class that was introduced by two of the three
sheets and uses this spacing class in all of them.

To fully fix the footer spacing, overscroll is now extended from 8dip
to 64dip as shown in the mocks in the linked bug.

Bug: 997611
Change-Id: I33f813b174a887f496413240efeebdaea3844426
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768466Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690344}