Make FadingEdgeScrollView more generic

* Move this class from out of the payments directory into the widget directory.

* Make the dialog edges consistent with Material Design and use lines that fade
  in/out instead of shadows that fade in/out.

Screenshots:
https://drive.google.com/corp/drive/folders/0B7c8ZkXVwskDRkpvdDhubHVnRmc

BUG=701859

Review-Url: https://codereview.chromium.org/2761583002
Cr-Commit-Position: refs/heads/master@{#458198}
10 files changed