[cleanup] Fix use of non-Isolate ToString in Adtracker

Gets the Isolate via ToIsolate(LocalFrame*).

Change-Id: I8e6fcb1043d5774483cd9533d848ae5d0fcf6179
Reviewed-on: https://chromium-review.googlesource.com/1151195Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578580}

third_party/blink/renderer/core/frame/ad_tracker.cc

@@ -76,7 +76,7 @@ void AdTracker::Will(const probe::CallFunction& probe) {
       probe.function->GetScriptOrigin().ResourceName();
   String script_url;
   if (!resource_name.IsEmpty())
-    script_url = ToCoreString(resource_name->ToString());
+    script_url = ToCoreString(resource_name->ToString(ToIsolate(local_root_)));
   WillExecuteScript(probe.context, script_url);
 }