Fix Chrome branded Chrome OS build.

TBR: sky@chromium.org
Bug: none
Change-Id: Iea3da6ecd4ee4c982ec9ffd11e8433d2c8a706dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436910Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811374}

ash/resources/vector_icons/BUILD.gn

@@ -318,7 +318,7 @@ aggregate_vector_icons2("ash_vector_icons") {
   ]
 
   if (is_chrome_branded) {
-    icons += [
+    sources += [
       "system_menu_cast_device.icon",
       "system_menu_cast_education.icon",
       "system_menu_cast_hangout.icon",