[Explore Sites] Fix render tests for other builders.

Some tablets failed because scrolling to a position did not guarantee
that position would be the first visible, since the catalog is not
big enough on a tablet.

Also switches scrollToPosition -> scrollToPositionWithOffset to match
behavior of moving a tile to the top.

Bug: 953254
Change-Id: Id7671a73e4ce16cc834ac66daa25369205ed4eee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570189
Reviewed-by: Cathy Li <chili@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652173}
3 files changed