Added a UMA stat to for new/old BITS jobs in the component updater.

The component updater can look into the BITS queue to see if
a job already exist to download a given component. This stat
provides an insight of whether this optimization is useful.

TBR=waffles

Bug: 737730
Change-Id: I56b31ad7b382197970b2dc49e8b42a7620a8e943
Reviewed-on: https://chromium-review.googlesource.com/564178Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485352}

components/update_client/background_downloader_win.cc

@@ -675,8 +675,6 @@ bool BackgroundDownloader::OnStateTransferring() {
   return false;
 }
 
-// Creates or opens a job for the given url and queues it up. Tries to
-// install a job observer but continues on if an observer can't be set up.
 HRESULT BackgroundDownloader::QueueBitsJob(const GURL& url,
                                            ComPtr<IBackgroundCopyJob>* job) {
   DCHECK(task_runner()->RunsTasksInCurrentSequence());
@@ -699,6 +697,9 @@ HRESULT BackgroundDownloader::QueueBitsJob(const GURL& url,
     return hr;
   }
 
+  const bool is_new_job = hr == S_OK;
+  UMA_HISTOGRAM_BOOLEAN("UpdateClient.BackgroundDownloaderNewJob", is_new_job);
+
   hr = local_job->Resume();
   if (FAILED(hr)) {
     CleanupJob(local_job);
@@ -717,7 +718,7 @@ HRESULT BackgroundDownloader::CreateOrOpenJob(const GURL& url,
                     bits_manager_, &jobs);
   if (SUCCEEDED(hr) && !jobs.empty()) {
     *job = jobs.front();
-    return hr;
+    return S_FALSE;
   }
 
   ComPtr<IBackgroundCopyJob> local_job;

components/update_client/background_downloader_win.h

@@ -87,6 +87,9 @@ class BackgroundDownloader : public CrxDownloader {
   void StartTimer();
   void OnTimer();
 
+  // Creates or opens a job for the given url and queues it up. Returns S_OK if
+  // a new job was created or S_FALSE if an existing job for the |url| was found
+  // in the BITS queue.
   HRESULT QueueBitsJob(const GURL& url,
                        Microsoft::WRL::ComPtr<IBackgroundCopyJob>* job);
   HRESULT CreateOrOpenJob(const GURL& url,

tools/metrics/histograms/histograms.xml

@@ -81111,6 +81111,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UpdateClient.BackgroundDownloaderNewJob" enum="Boolean">
+  <owner>sorin@chromium.org</owner>
+  <summary>
+    True if a new BITS job was created to download the url, or false, if an
+    existing job was found in the BITS queue. The metric is recorded when
+    the component updater background downloader is asked to download a CRX.
+  </summary>
+</histogram>
+
 <histogram name="UpdateEngine.Attempt.ConnectionType"
     enum="UpdateEngineConnectionType">
   <owner>zeuthen@chromium.org</owner>