MD Settings: Fix size and padding of expand button
This CL includes some cleanup to make the vars used for icon padding
consistent between cr_elements and settings UI.
BUG=
674451
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
Committed:
https://crrev.com/3b3d36ba882ca4a8c55a8be080d672abd83d5bea
Cr-Commit-Position: refs/heads/master@{#439531}