[filesapp] Add DevToolsListener verbose message logging

Add vmodule logging to log the content of CDP messages received by the
devtools listener from the content::DevToolsAgentHost. Add the command
line switch --vmodule=devtools_listener=2 to enable vmodule logs.

Tbr: benreich
Bug: 1113941
Change-Id: I3b91bad8dba44183a1f4a0a0ae8618311f734a98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483707Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818329}

chrome/browser/chromeos/file_manager/devtools_listener.cc

@@ -247,6 +247,9 @@ void DevToolsListener::DispatchProtocolMessage(
   if (!navigated_)
     return;
 
+  if (VLOG_IS_ON(2))
+    VLOG(2) << SpanToStringPiece(message);
+
   std::unique_ptr<base::DictionaryValue> value = base::DictionaryValue::From(
       base::JSONReader::ReadDeprecated(SpanToStringPiece(message)));
   CHECK(value);