Bug 1470947 - Part 3: Ensure tab throbbers are always hidden with browser.tabs.hideThrobber r=mconley

The tab throbbers were being shown in the new all tabs menu. If this pref
is set then we never want them to be visible, so using !important to fix
this and future issues.

MozReview-Commit-ID: DHRrCAKEwnA

--HG--
extra : rebase_source : 9999e458bf23c15cbecdb49942a592f45daffa1e

browser/base/content/browser.css

@@ -206,7 +206,7 @@ panelview[mainview] > .panel-header {
 @supports -moz-bool-pref("browser.tabs.hideThrobber") {
   .tab-throbber,
   .tab-throbber-fallback {
-    display: none;
+    display: none !important;
   }
 }
 %endif