Tracing: fix missing comma between trace chunks written to file

content/browser/tracing/tracing_controller_impl.cc

Index: content/browser/tracing/tracing_controller_impl.cc
diff --git a/content/browser/tracing/tracing_controller_impl.cc b/content/browser/tracing/tracing_controller_impl.cc
index 73c4857ec74180ae0f00dbd0dd6b4f9a03c7563f..2362a1576377fec91a58f90a3beaf50315ef0cce 100644
--- a/content/browser/tracing/tracing_controller_impl.cc
+++ b/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_));