OOBE - Remove background from EULA ToS

Injects additional CSS into the EULA webview to set the background
to transparent. This allows Chrome OS to show the inner webview shadows,
the scrolling shadows, and the rounded corners on the EULA webview.

Fixed: 1061030, 1065275
Change-Id: Ib58fe9512926f6267344d82d0c5692c9ab3610a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216079Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Renato Silva <rrsilva@google.com>
Cr-Commit-Position: refs/heads/master@{#792620}

chrome/browser/resources/chromeos/login/oobe_screen_eula.js

@@ -23,7 +23,11 @@ login.createScreen('EulaScreen', 'eula', function() {
     code: `body * {
           font-family: Roboto, sans-serif !important;
           font-size: 13px !important;
+          background-color: transparent !important;
           line-height: 20px !important;}
+         body {
+          background-color: transparent !important;
+         }
          body h2 {
            font-size: 15px !important;
            line-height: 22px !important;}`