Make WebContentsImpl Loading event show up in perfetto

Changes WebContentsImpl Loading trace event to be a nestable trace event
which means that it will show up in perfetto which doesn't support
non-nested async events.

Bug: 1142477
Change-Id: Ic8a279b5b0c99a02d0cde0a05ffcd45bfef2c827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494863
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820826}

content/browser/web_contents/web_contents_impl.cc

@@ -6237,14 +6237,14 @@ void WebContentsImpl::LoadingStateChanged(bool to_different_document,
 
   std::string url = (details ? details->url.possibly_invalid_spec() : "NULL");
   if (is_loading) {
-    TRACE_EVENT_ASYNC_BEGIN2("browser,navigation", "WebContentsImpl Loading",
-                             this, "URL", url, "Main FrameTreeNode id",
-                             GetFrameTree()->root()->frame_tree_node_id());
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN2(
+        "browser,navigation", "WebContentsImpl Loading", this, "URL", url,
+        "Main FrameTreeNode id", GetFrameTree()->root()->frame_tree_node_id());
     observers_.ForEachObserver(
         [&](WebContentsObserver* observer) { observer->DidStartLoading(); });
   } else {
-    TRACE_EVENT_ASYNC_END1("browser,navigation", "WebContentsImpl Loading",
-                           this, "URL", url);
+    TRACE_EVENT_NESTABLE_ASYNC_END1(
+        "browser,navigation", "WebContentsImpl Loading", this, "URL", url);
     observers_.ForEachObserver(
         [&](WebContentsObserver* observer) { observer->DidStopLoading(); });
   }