Scroll time columns to the 2nd half to allow users to scroll up.

Since in the time picker each column has the list of cells doubled we can
start with each column scrolled to the middle in order to allow scrolling up.

The new behavior is validated by time-picker-appearance-wheel.html and other
existing tests.

Bug: 1021343
Change-Id: I65b04f3f1fec79c2efc75f54a7798726c6ab1200
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1907522
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714401}