[NTP] Enable scrolling for the voice search dialog

Use absolute instead of fixed positioning for the voice search dialog in
order to enable scrolling.

Bug: 865831
Change-Id: I43505223301c77e6ac8502a1f505a0d4d0def6ab
Reviewed-on: https://chromium-review.googlesource.com/c/1416951
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623812}
2 files changed