Call CSD whitelist checking on UI thread and record UMA

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 df0613de8423b8275326dd89e44e49417104393a..a5ab33873702e35eee5ed550fa430a7255e34a85 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -45985,6 +45985,21 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.PasswordReuse.MainFrameMatchCsdWhitelist"

[vakh (use Gerrit instead), 2017/03/02 01:01:01]

Nit: You should consider adding PasswordManager.PasswordReuse as a prefix, if
that's possible.

[Jialiu Lin, 2017/03/02 01:33:14]

Agree. I thought of that. I guess I'll defer this to dvadym@, since he is the
owner of all the other PasswordManager.PasswordReuse metrics. 

+    enum="Boolean">
+  <owner>dvadym@chromium.org</owner>
+  <owner>jialiul@chromium.org</owner>
+  <owner>nparker@chromium.org</owner>
+  <summary>
+    A password reuse is an event when the user typed a string that is equal to a
+    saved password on another domain, and this saved password is called reused
+    password.
+
+    This metric shows whether a password reuse happens on a page whose main
+    frame url matches CSD whitelist.
+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.PasswordReuse.NumberOfMatches"
     units="credentials">
   <owner>dvadym@chromium.org</owner>