Tracing: fix missing comma between trace chunks written to file

TBR=nduca,dsinclair
BUG=415388

Review URL: https://codereview.chromium.org/583583002

Cr-Commit-Position: refs/heads/master@{#295447}

content/browser/tracing/tracing_controller_impl.cc

@@ -68,7 +68,9 @@ class FileTraceDataSink : public TracingController::TraceDataSink {
 
   void AddTraceChunkOnFileThread(
       const scoped_refptr<base::RefCountedString> chunk) {
-    if (!OpenFileIfNeededOnFileThread())
+    if (file_ != NULL)
+      fputc(',', file_);
+    else if (!OpenFileIfNeededOnFileThread())
       return;
     ignore_result(fwrite(chunk->data().c_str(), strlen(chunk->data().c_str()),
         1, file_));