Add trace events for ThreadedDataProvider

This change adds trace events to ThreadedDataProvider. Even though the
feature is still disabled by default, having trace events helps us
investigate the behavior locally.

BUG=581992, 277124
R=kinuko@chromium.org

Review URL: https://codereview.chromium.org/1640283004

Cr-Commit-Position: refs/heads/master@{#372054}

content/child/threaded_data_provider.cc

@@ -112,6 +112,7 @@ void DataProviderMessageFilter::OnReceivedData(int request_id,
                                                int data_offset,
                                                int data_length,
                                                int encoded_data_length) {
+  TRACE_EVENT0("loader", "DataProviderMessageFilter::OnReceivedData");
   DCHECK(io_task_runner_->BelongsToCurrentThread());
   background_thread_.TaskRunner()->PostTask(
       FROM_HERE,
@@ -303,6 +304,8 @@ void ThreadedDataProvider::OnReceivedDataOnBackgroundThread(
 void ThreadedDataProvider::OnReceivedDataOnForegroundThread(
     const char* data, int data_length, int encoded_data_length) {
   DCHECK(ChildThreadImpl::current());
+  TRACE_EVENT0("loader",
+               "ThreadedDataProvider::OnReceivedDataOnForegroundThread");
 
   background_thread_.TaskRunner()->PostTask(
       FROM_HERE, base::Bind(&ThreadedDataProvider::ForwardAndACKData,