[Tracing] Add process label for SubFrames in tracing

With the addition of site isolation, we now can have render
processes that are not assigned to a top level page, hence
they do not get a descriptive title in tracing.

This CL adds information about all the hosted subframes to
the process label in tracing.

Bug: chromium:828072
Change-Id: Ia6f617dc936f0c92183684faee39c65447bfd37c
Reviewed-on: https://chromium-review.googlesource.com/990553Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Fadi Meawad <fmeawad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547700}

content/renderer/render_frame_impl.cc

@@ -4427,6 +4427,17 @@ void RenderFrameImpl::DidReceiveTitle(const blink::WebString& title,
     base::string16 shortened_title = title16.substr(0, kMaxTitleChars);
     Send(new FrameHostMsg_UpdateTitle(routing_id_,
                                       shortened_title, direction));
+  } else {
+    // Set process title for sub-frames in traces.
+    GURL loading_url = GetLoadingUrl();
+    if (!loading_url.host().empty() &&
+        loading_url.scheme() != url::kFileScheme) {
+      std::string subframe_title = "Subframe: " + loading_url.scheme() +
+                                   url::kStandardSchemeSeparator +
+                                   loading_url.host();
+      base::trace_event::TraceLog::GetInstance()->UpdateProcessLabel(
+          routing_id_, subframe_title);
+    }
   }
 
   // Also check whether we have new encoding name.