[NTP] Fix scrolling on background customization dialogs

If a customize background dialog is opened and the space
key is pressed the main page shouldn't scroll. If the mouse
is used to scroll the page the dialog should stay centered
while the page scrolls in the background (this is consistent
with the custom links dialogs).

Screencast: http://go/scrcast/NTY3MzY0NjkyODQyOTA1Nnw1MzY0N2JhNS1hZA

Bug: 908381, 899730
Change-Id: I71e99f6d4481c9e61d3f98d0ff23f4d059a21464
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501934
Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637593}
2 files changed