Add UMA to record why chrome shows generic warning

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e90336494d96df158b3cc62e6eb612df59bc7b23..2ab2e09e3c0d8a63a80cea99a33872780505cd10 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10602,6 +10602,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.DangerousFile.Reason" enum="DangerousFile.Reason">
+  <owner>jialiul@chromium.org</owner>
+  <owner>nparker@chromium.org</owner>
+  <summary>
+    Indicates why a download is marked as DANGEROUS_FILE. Grouped by reason,
+    such as Safe Browsing (SB) service is not available, and SB returns UNKOWN
+    or SAFE verdict. The sum of all reasons should roughly equal to the
+    DANGEROUS_FILE bucket count in Download.DownloadWarningShown.
+  </summary>
+</histogram>
+
 <histogram name="Download.DangerousFile.UserDiscard"
     enum="DownloadItem.DangerousFileType">
   <owner>asanka@chromium.org</owner>
@@ -70410,6 +70421,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="Clock Changed"/>
 </enum>
 
+<enum name="DangerousFile.Reason" type="int">
+  <int value="0" label="Safe Browsing is not available"/>
+  <int value="1" label="Safe Browsing returns UNKOWN"/>
+  <int value="2" label="Safe Browsing returns SAFE"/>
+</enum>
+
 <enum name="DarkResumeScanRetryResult" type="int">
   <int value="0" label="Not Connected"/>
   <int value="1" label="Connected"/>