Modify the button bar style to have wider hit-area for arrow button.
As is reported in the bug, the arrow button width is too narrow.
This CL specifies its size to a square of kButtonSize, and
modifies some constants to keep the same visual alignment.
BUG=517974
R=yoshiki@chromium.org, dewittj@chromium.org
TEST=checked manually, verified graphical position doesn't change
with screenshot comparison
Review URL: https://codereview.chromium.org/
1287163007
Cr-Commit-Position: refs/heads/master@{#344320}