Check whether ToString()'s result is empty in AdTracker::Will()

While AdTracker::Will() already checks if |resource_name| is empty, in
rare situations, the subsequent call to ToString() can still return an
empty result. This is the cause of a rare crash. To fix this, we check
whether the result is empty; if it is, we treat it in the same was as an
empty |resource_name|.

Bug: 1086832
Change-Id: Ie9d3a865c5a588a34f94034d55bba1ae67e6895e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225282Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775833}

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

@@ -158,9 +158,12 @@ void AdTracker::Will(const probe::CallFunction& probe) {
       probe.function->GetScriptOrigin().ResourceName();
   String script_url;
   if (!resource_name.IsEmpty()) {
-    script_url = ToCoreString(
-        resource_name->ToString(ToIsolate(local_root_)->GetCurrentContext())
-            .ToLocalChecked());
+    v8::MaybeLocal<v8::String> resource_name_string =
+        resource_name->ToString(ToIsolate(local_root_)->GetCurrentContext());
+    // Rarely, ToString() can return an empty result, even if |resource_name|
+    // isn't empty (crbug.com/1086832).
+    if (!resource_name_string.IsEmpty())
+      script_url = ToCoreString(resource_name_string.ToLocalChecked());
   }
   WillExecuteScript(probe.context, script_url);
 }