[NTP] Add pressed state to back arrow

http://go/scrcast/NTA5MjU0NjM3MTkxMTY4MHw4ZWNkMjkzNS0xMg

Also fix a padding issue in RTL mode, introduced when the scrollbar
was added.

Bug: 859371
Change-Id: I8745af5191f117e712eceaba5f09a68f9166d19b
Reviewed-on: https://chromium-review.googlesource.com/1144317
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576729}
2 files changed