Add UMA metric to track parallel download requests stats.

content/browser/download/parallel_download_job.cc

Index: content/browser/download/parallel_download_job.cc
diff --git a/content/browser/download/parallel_download_job.cc b/content/browser/download/parallel_download_job.cc
index b9df978537da94ee946704a3c3d0cb3d82dfa233..1301a6d3f8e4797c9a1741fe756199400f32c215 100644
--- a/content/browser/download/parallel_download_job.cc
+++ b/content/browser/download/parallel_download_job.cc
@@ -6,6 +6,7 @@
 
 #include "base/memory/ptr_util.h"
 #include "content/browser/download/download_create_info.h"
+#include "content/browser/download/download_stats.h"
 #include "content/browser/download/parallel_download_utils.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/storage_partition.h"
@@ -102,6 +103,7 @@ void ParallelDownloadJob::OnByteStreamReady(
     std::unique_ptr<ByteStreamReader> stream_reader) {
   bool success = DownloadJob::AddByteStream(std::move(stream_reader),
                                             worker->offset(), worker->length());
+  RecordParallelDownloadAddStreamSuccess(success);
 
   // Destroy the request if the sink is gone.
   if (!success) {
@@ -170,21 +172,31 @@ void ParallelDownloadJob::BuildParallelRequests() {
 void ParallelDownloadJob::ForkSubRequests(
     const DownloadItem::ReceivedSlices& slices_to_download) {
   bool initial_request_skipped = false;
+  int requests_num = 0;
+  int early_requests_num = 0;
+
   for (auto it = slices_to_download.begin(); it != slices_to_download.end();
        ++it) {
     // Create requests for holes before the |initial_request_offset_|.
     if (it->offset < initial_request_offset_) {
       CreateRequest(it->offset, it->received_bytes);
+      early_requests_num++;
+      requests_num++;
       continue;
     }
 
     // Assume the first slice to download after |initial_request_offset_| will
     // be handled by the initial request.
-    if (initial_request_skipped)
+    if (initial_request_skipped) {
       CreateRequest(it->offset, it->received_bytes);
-    else
+      requests_num++;
+    } else
       initial_request_skipped = true;
   }
+
+  RecordParallelDownloadRequestCount(requests_num + 1);
+  if (early_requests_num > 0)
+    RecordParallelDownloadEarlyRequests(early_requests_num);
 }
 
 void ParallelDownloadJob::CreateRequest(int64_t offset, int64_t length) {