Added 'Not Started' to DownloadFileResult histogram.

This histogram is logged when the user closes Download Manager UI
without starting the download.

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4e7912e312b1eccbb839207160ed82cbb3d95577
Reviewed-on: https://chromium-review.googlesource.com/981461Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546124}

ios/chrome/browser/download/download_manager_metric_names.h

@@ -34,6 +34,8 @@ enum class DownloadFileResult {
   // In progress download did no finish because the tab was closed or user has
   // quit the app.
   Other = 3,
+  // The user closed Download Manager UI without starting the download.
+  NotStarted = 4,
   Count
 };
 

ios/chrome/browser/ui/download/download_manager_coordinator.mm

@@ -227,6 +227,9 @@ class UnopenedDownloadsTracker : public web::DownloadTaskObserver {
 - (void)downloadManagerViewControllerDidClose:
     (DownloadManagerViewController*)controller {
   if (_downloadTask->GetState() != web::DownloadTask::State::kInProgress) {
+    UMA_HISTOGRAM_ENUMERATION("Download.IOSDownloadFileResult",
+                              DownloadFileResult::NotStarted,
+                              DownloadFileResult::Count);
     [self cancelDownload];
     return;
   }

ios/chrome/browser/ui/download/download_manager_coordinator_unittest.mm

@@ -305,6 +305,10 @@ TEST_F(DownloadManagerCoordinatorTest, Close) {
   EXPECT_EQ(0U, base_view_controller_.childViewControllers.count);
   EXPECT_FALSE(coordinator_.downloadTask);
   EXPECT_EQ(web::DownloadTask::State::kCancelled, task.GetState());
+  histogram_tester_.ExpectUniqueSample(
+      "Download.IOSDownloadFileResult",
+      static_cast<base::HistogramBase::Sample>(DownloadFileResult::NotStarted),
+      1);
 }
 
 // Tests presenting Install Google Drive dialog. Coordinator presents StoreKit

tools/metrics/histograms/enums.xml

@@ -9961,10 +9961,18 @@ Called by update_net_error_codes.py.-->
 </enum>
 
 <enum name="DownloadFileResult">
-  <int value="0" label="Completed"/>
-  <int value="1" label="Canceled"/>
-  <int value="2" label="Failure"/>
-  <int value="3" label="Other"/>
+  <int value="0" label="Completed">Download has successfully completed.</int>
+  <int value="1" label="Canceled">
+    In progress download was cancelled by the user.
+  </int>
+  <int value="2" label="Failure">Download has completed with error.</int>
+  <int value="3" label="Other">
+    In progress download did no finish because the tab was closed or user has
+    quit the app.
+  </int>
+  <int value="4" label="Not Started">
+    The user closed Download Manager UI without starting the download.
+  </int>
 </enum>
 
 <enum name="DownloadFunctions">