chrome://signin-reauth: fix illustration size in dark mode

The reauth dialog CSS overrides the background illustration in dark
mode using the `background` shorthand CSS property which sets all
background style properties at once. In this case, `background-size`
attribute gets implicitly overridden with a default value.

This behavior isn't desired, so this CL uses `background-image` CSS
property instead of the `background` shortcut.

Bug: 1114701
Change-Id: I32232b540b7feb3d76c524af589497382d6f12f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346293Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796999}

chrome/browser/resources/signin/signin_reauth/signin_reauth_app.html

@@ -13,7 +13,7 @@
   }
 
   #illustration {
-    background: url(./images/signin_reauth_illustration.svg);
+    background-image: url(./images/signin_reauth_illustration.svg);
     background-size: 100% 100%;
     height: 100%;
     position: absolute;
@@ -23,7 +23,7 @@
 
   @media (prefers-color-scheme: dark) {
     #illustration {
-      background: url(./images/signin_reauth_illustration_dark.svg);
+      background-image: url(./images/signin_reauth_illustration_dark.svg);
     }
   }