Add yet another download_complete variant needle

Sometimes this is pressed down, sometimes it isn't, sometimes
it's blue, I'm losing track...

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/firefox/download_complete-20180514.json

@@ -0,0 +1,16 @@
+{
+  "properties": [],
+  "tags": [
+    "browser_download_complete",
+    "firefox_download_complete"
+  ],
+  "area": [
+    {
+      "ypos": 108,
+      "width": 17,
+      "type": "match",
+      "height": 19,
+      "xpos": 894
+    }
+  ]
+}