feat: Hide PiP urlbar button via a pref instead of overriding styles, b=no-bug, c=common

prefs/firefox/urlbar.yaml

@@ -22,6 +22,9 @@
 - name: browser.urlbar.untrimOnUserInteraction.featureGate
   value: true
 
+- name: media.videocontrols.picture-in-picture.urlbar-button.enabled
+  value: false
+
 # Enable private suggestions
 - name: browser.search.suggest.enabled
   value: false

src/zen/common/styles/zen-omnibox.css

@@ -740,7 +740,6 @@
 #reader-mode-button,
 #urlbar-go-button,
 #star-button-box,
-#picture-in-picture-button,
 #pageActionButton:not([open]) {
   display: none !important;
 }