Added missing dependency to ui/download/BUILD.gn.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ic7c34b7594afea46412b2ac861ced2b60d01d7c7
Reviewed-on: https://chromium-review.googlesource.com/822511Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523497}

ios/chrome/browser/ui/download/BUILD.gn

@@ -55,6 +55,7 @@ source_set("unit_tests") {
     "//ios/chrome/browser/download:test_support",
     "//ios/chrome/browser/infobars",
     "//ios/chrome/browser/store_kit",
+    "//ios/chrome/browser/ui:ui_util",
     "//ios/chrome/browser/web:test_support",
     "//ios/chrome/test:test_support",
     "//ios/chrome/test/fakes",