Add UMA to record why chrome shows generic warning

chrome/browser/download/chrome_download_manager_delegate.cc

Index: chrome/browser/download/chrome_download_manager_delegate.cc
diff --git a/chrome/browser/download/chrome_download_manager_delegate.cc b/chrome/browser/download/chrome_download_manager_delegate.cc
index c3aa96086756d847e1c5d6509157a4893bb51e3e..4f5f4b4f72b19640d312401bd3b819e2a258928c 100644
--- a/chrome/browser/download/chrome_download_manager_delegate.cc
+++ b/chrome/browser/download/chrome_download_manager_delegate.cc
@@ -11,6 +11,7 @@
 #include "base/callback.h"
 #include "base/files/file_util.h"
 #include "base/macros.h"
+#include "base/metrics/histogram.h"
 #include "base/rand_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
@@ -181,6 +182,16 @@ std::string GetMimeType(const base::FilePath& path) {
   return mime_type;
 }
 
+// Reason for why danger type is DOWNLOAD_DANGER_TYPE_DANGEROUS_FILE.
+// Used by "Download.DangerousFile.Reason" UMA metric.
+// Do not change the ordering or remove items.
+enum DangerousFileReason {
+  SB_NOT_AVAILABLE = 0,
+  SB_RETURNS_UNKOWN = 1,
+  SB_RETURNS_SAFE = 2,
+  DANGEROUS_FILE_REASON_MAX
+};
+
 }  // namespace
 
 ChromeDownloadManagerDelegate::ChromeDownloadManagerDelegate(Profile* profile)
@@ -337,6 +348,8 @@ bool ChromeDownloadManagerDelegate::IsDownloadReadyForCompletion(
                << "() SB service disabled. Marking download as DANGEROUS FILE";
       item->OnContentCheckCompleted(
           content::DOWNLOAD_DANGER_TYPE_DANGEROUS_FILE);
+      UMA_HISTOGRAM_ENUMERATION("Download.DangerousFile.Reason",
+                                SB_NOT_AVAILABLE, DANGEROUS_FILE_REASON_MAX);
       content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
                                        internal_complete_callback);
       return false;
@@ -686,16 +699,23 @@ void ChromeDownloadManagerDelegate::CheckClientDownloadDone(
       case DownloadProtectionService::UNKNOWN:
         // The check failed or was inconclusive.
         if (DownloadItemModel(item).GetDangerLevel() !=
-            DownloadFileType::NOT_DANGEROUS)
+            DownloadFileType::NOT_DANGEROUS) {
           danger_type = content::DOWNLOAD_DANGER_TYPE_DANGEROUS_FILE;
+          UMA_HISTOGRAM_ENUMERATION("Download.DangerousFile.Reason",
+                                    SB_RETURNS_UNKOWN,
+                                    DANGEROUS_FILE_REASON_MAX);
+        }
         break;
       case DownloadProtectionService::SAFE:
         // If this file type require explicit consent, then set the danger type
         // to DANGEROUS_FILE so that the user be required to manually vet
         // whether the download is intended or not.
         if (DownloadItemModel(item).GetDangerLevel() ==
-            DownloadFileType::DANGEROUS)
+            DownloadFileType::DANGEROUS) {
           danger_type = content::DOWNLOAD_DANGER_TYPE_DANGEROUS_FILE;
+          UMA_HISTOGRAM_ENUMERATION("Download.DangerousFile.Reason",
+                                    SB_RETURNS_SAFE, DANGEROUS_FILE_REASON_MAX);
+        }
         break;
       case DownloadProtectionService::DANGEROUS:
         danger_type = content::DOWNLOAD_DANGER_TYPE_DANGEROUS_CONTENT;