I misunderstood but there was already the implementation but it's behind the flag --secondary-ui-md flag. So we just have to add an option to default MD checkbox in notification settings.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b7521c49da40bcf4a08dfeccee9f158b7173aba6

commit b7521c49da40bcf4a08dfeccee9f158b7173aba6
Author: Tetsui Ohkubo <tetsui@chromium.org>
Date: Tue Oct 03 05:11:08 2017

Add Material Design option to ctor of views::Checkbox.

Material Design checkbox is available behind runtime flag
(--secondary-ui-md), but there was no way to access the feature
separately without enabling all Material Design secondary UI elements.

We need Material Design checkbox in Chrome OS notification settings.
Mock: https://crbug.com/726246#c8

TEST=manual
BUG= 767803 

Change-Id: I89dfbd3708cae64bd68183167a2088b5ca826856
Reviewed-on: https://chromium-review.googlesource.com/690057
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505950}
[modify] https://crrev.com/b7521c49da40bcf4a08dfeccee9f158b7173aba6/ui/views/controls/button/checkbox.cc
[modify] https://crrev.com/b7521c49da40bcf4a08dfeccee9f158b7173aba6/ui/views/controls/button/checkbox.h