[Merge to M60] Added UMA for extension permissions used in public sessions

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 c64fd65d31312a41e46ed95b8f28513aa0538276..6d3ab3f3c88c6a0cf5c95557ad0c6db4e771724b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -15720,6 +15720,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Enterprise.PublicSession.ExtensionPermissions"
+    enum="ExtensionPermission3">
+  <owner>isandrk@chromium.org</owner>
+  <owner>sduraisamy@chromium.org</owner>
+  <summary>
+    Extensions (and apps) are quite restricted in public sessions for security
+    and privacy concerns - some permissions are blocked which means the
+    extension itself is blocked (prevented from loading). This histogram counts
+    requested permissions on extension installation in public sessions (both
+    installed and blocked extensions).
+  </summary>
+</histogram>
+
 <histogram name="Enterprise.PublicSession.SessionLength" units="minutes">
   <owner>xiyuan@chromium.org</owner>
   <owner>sduraisamy@chromium.org</owner>