Do not search in all frames.

Chrome has never searched in all frames, so this is no loss.

However, find on Firefox Quantum is now broken with the "all frames"
flag set.

Fixes #2962.

H/t: @gdh1995 (#2962 (comment)).

content_scripts/mode_find.coffee

@@ -177,7 +177,7 @@ class FindMode extends Mode
       document.removeEventListener("selectionchange", @restoreDefaultSelectionHighlight, true)
 
     try
-      result = window.find(query, options.caseSensitive, options.backwards, true, false, true, false)
+      result = window.find(query, options.caseSensitive, options.backwards, true, false, false, false)
     catch # Failed searches throw on Firefox.
 
     # window.find focuses the |window| that it is called on. This gives us an opportunity to (re-)focus