Remove scrollbar in Assistant Voice Match screen

This scrollbar showed up on small devices when retraining the voice match
(settings -> Search and Assistant -> Google Assistant -> "Ok Google"
-> Voice match -> Retrain).

After investigating I also noticed the header on every Assistant screen was
12px lower than the headers on the other OOBE screens.

Fixing that also fixed the scrollbar issue, as now the screen is small enough
to fit.

Note that the OOBE spec does not specify the height between the top-left icon
and the header, but it is set to 20px here so I copied that:
https://osscs.corp.google.com/chromium/chromium/src/+/master:chrome/browser/resources/chromeos/login/oobe_dialog.css;l=30?q=oobe-title&ss=chromium

Bug: b/147691810
Change-Id: I1fa872c16e4a2ba55f06cb8316b1511ca5fbb28d
Tests: Manually deployed and tested
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001162Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Cr-Commit-Position: refs/heads/master@{#732642}

chrome/browser/resources/chromeos/assistant_optin/assistant_shared_styles.css

@@ -46,7 +46,7 @@ webview {
   color: var(--google-grey-900);
   font-family: 'Google Sans', Roboto, sans-serif;
   font-size: 28px;
-  padding: 32px 0 16px 0;
+  padding: 20px 0 16px 0;
 }
 
 .sub-title {