HTMLViewer: Log JS messages to console

BUG=

Review URL: https://codereview.chromium.org/1005413002

Cr-Commit-Position: refs/heads/master@{#320786}

mojo/services/html_viewer/html_document.cc

@@ -276,6 +276,8 @@ void HTMLDocument::didAddMessageToConsole(
     const blink::WebString& source_name,
     unsigned source_line,
     const blink::WebString& stack_trace) {
+  VLOG(1) << "[" << source_name.utf8() << "(" << source_line << ")] "
+          << message.text.utf8();
 }
 
 void HTMLDocument::didNavigateWithinPage(