Adds ui_test_pak to html_viewer's deps

We currently need it, so we should have a dependency on it.

R=erg@chromium.org

Review URL: https://codereview.chromium.org/1032733002

Cr-Commit-Position: refs/heads/master@{#322103}

mojo/services/html_viewer/BUILD.gn

@@ -102,6 +102,7 @@ source_set("lib") {
     "//third_party/mojo_services/src/surfaces/public/interfaces",
     "//ui/gfx/geometry",
     "//ui/native_theme",
+    "//ui/resources:ui_test_pak",
     "//url",
   ]