Add a histogram for a number of HTTP passwords migrated to HTTPS.

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 5d9331759593f70127c81daac7e393bdafac2fa1..5720b558e5a6a4de7e2ef178dca73007a21bc05f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -44941,6 +44941,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.HttpPasswordMigrationCount">

[Ilya Sherman, 2017/01/23 20:22:58]

Sorry for not being clear enough previously.  I meant: Please add an attribute
like units="saved credential entries".

+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    Number of HTTP saved passwords that were migrated to HTTPS. Recorded on
+    password form load.

[Ilya Sherman, 2017/01/23 20:22:58]

Hmm, is this recorded on *every* password form load?  That would suggest this
histogram would have mostly zeroes, which I'm assuming isn't the case.  Could
you please provide some more detail to clarify when this is recorded?

+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.ImportedPasswordsPerUserInCSV">
   <owner>xunlu@chromium.org</owner>
   <owner>vabr@chromium.org</owner>