Revert "ChromeOS WebUI: replace paper-button with cr-button"

This reverts commit f333ca63bbd0a1fbd78f8867b8fc899e4767b69b.

Reason for revert: CL seems to cause {browser_tests, viz_browser_tests} failure for "Linux ChromiumOS MSan Tests" for

EphemeralUserOobeTestImpl/EphemeralUserOobeTest.RegularEphemeralUser/1
...

Original change's description:
> ChromeOS WebUI: replace paper-button with cr-button
> 
> Bug: 967793
> Change-Id: Ia8dbde98cb791b7b87fe3bd69287fdc64bfe2fc6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1640605
> Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
> Reviewed-by: James Cook <jamescook@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#667401}

TBR=jamescook@chromium.org,aee@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 967793
Change-Id: I1e5e5555b7ba3e9220b6d2f502c8c62f4e3092ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1651106
Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667497}