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[Autofill Assistant] Make transparent areas look nicer.

This change adds a blue line around transparent areas and adds a fade
animation when the transparent area appears or disappears.

Screenshot:
https://screenshot.googleplex.com/kAH8AHKk2du.png

Animation:
https://drive.google.com/open?id=1E_X48t1Kl-z2JAJpWyVtuCP8tmzcrnEz

Bug: b/125245127
Change-Id: I95ba459c6420620b64225c9043e00ce2dcfd25d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497138
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637645}