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These buttons were implemented mostly with the border-image CSS property
and reused images that were shared between WebUI and Views. Views no longer
uses those images. Instead replacing with a full CSS implementation (without
using border-image), allows removing those images.

The images are no longer used anywhere after this CL, and will be fully
removed in follow up CL.

Bug: 522168
Test: Open the feedback app. Cancel and Send buttons should look identical (AFAICT) as before.
Change-Id: I6a5d9625f1f042b1fb99a4247b523215f291fc34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639341
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666137}