Fix broken crostini_upgrader colors.

https://chromium-review.googlesource.com/c/chromium/src/+/2126408
renamed a color which broke existing clients.

This CL fixes the crostini_upgrader.

Bug: 1070423
Change-Id: I3015451bd25376ad981c160a2551b94fcd197718
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147130Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758818}

chrome/browser/resources/chromeos/crostini_upgrader/app.html

@@ -14,7 +14,7 @@
   }
 
   #icon {
-    fill: var(--cros-default-button-color);
+    fill: var(--cros-default-icon-color-prominent);
     height: 32px;
     margin-top: 28px;
     width: 32px;
@@ -62,7 +62,7 @@
   }
 
   paper-progress {
-    --paper-progress-active-color: var(--cros-default-button-color);
+    --paper-progress-active-color: var(--cros-default-icon-color-prominent);
     --paper-progress-container-color: rgba(var(--google-blue-600-rgb), .24);
     margin-top: 36px;
     width: 100%;