[CrOS Wallpaper App] Add ripple effect on selected category.

Currently there is no ripple effect when the user clicks on a wallpaper
category.  This CL adds that ripple effect.

Change-Id: Id7cf9fb12f26bbc4165919eb1f20c90210046022
Bug: 835194
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507283
Reviewed-by: May Lippert <maybelle@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638914}
2 files changed