This CL contains the following changes:
 - Add a background image behind the caption preview text
 - Move the preview section to the top of the page
 - Remove the top padding of the preview section
 - Replace the opacity sliders with dropdowns

Bug: 1009561
Change-Id: I659bccfbd5bac27f63afba0e4f899fb94a2e5d84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1831943Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#703846}