Bug 1269457 - Only set breakpoints for pending bp actors. r=ejpbruel

The special code path carved out in bug 1225160 dropped the `actor.isPending`
check which causes many, many attempts to set a breakpoint on every new source
notification, leading to a very slow debugging experience.

MozReview-Commit-ID: A3pnHzh5eeh

devtools/server/actors/script.js

@@ -2014,7 +2014,9 @@ const ThreadActor = ActorClass({
       // debugger, and carefully avoid the use of unsafeSynchronize in this
       // function when source maps are disabled.
       for (let actor of bpActors) {
-        actor.originalLocation.originalSourceActor._setBreakpoint(actor);
+        if (actor.isPending) {
+          actor.originalLocation.originalSourceActor._setBreakpoint(actor);
+        }
       }
     }