Added UMA statistics for change passwords in the Password Manager

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 0fb79261fea28362938b2a86a7eb0a8a37c3823a..7137fe39366904788e312e976735c092e0253429 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29356,6 +29356,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PasswordManager.UpdatePasswordSubmissionEvent"
+    enum="UpdatePasswordSubmissionEvent">
+  <owner>dvadym@chromium.org</owner>
+  <owner>vasilii@chromium.org</owner>
+  <summary>
+    Which password update event was happened and what did the user do with the

[Ilya Sherman, 2015/08/20 00:30:51]

nit: "was happened" -> "happened"

[dvadym, 2015/08/20 11:10:11]

Done.

+    password update bubble.
+  </summary>
+</histogram>
+
 <histogram name="PasswordManager.UserStoredPasswordWithInvalidSSLCert"
     enum="Boolean">
   <owner>xunlu@chromium.org</owner>
@@ -71952,6 +71962,21 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="2" label="Forced Full"/>
 </enum>
 
+<enum name="UpdatePasswordSubmissionEvent" type="int">
+  <int value="0" label="NO_ACCOUNTS_CLICKED_UPDATE"/>

[Ilya Sherman, 2015/08/20 00:30:51]

What does this case mean?  What accounts are in scope for this measurement, and
how does one update data if there aren't any accounts?

[dvadym, 2015/08/20 11:10:11]

It means that there is no accounts stored in the Password Manager, but the user
has an account on some site and changed password for it. In this case currently
we are going to propose to save password without username and then save username
on the first successful sign-in. 

[Ilya Sherman, 2015/08/21 01:24:23]

This might be a case where I'd actually recommend using the expanded form for an
enum, with descriptions of each enum value, e.g.

  <int value="0" label="NO_ACCOUNTS_CLICKED_UPDATE">The user has no password
saved in Chrome for foo.com, but just changed her password on foo.com, and chose
to save the update password in Chrome's password manager.</int>

It can be a little shorter (or longer, if you'd prefer), but something like that
would really help me -- and probably other viewers -- to understand the
semantics of these labels on the dashboard.  (Admittedly, these descriptions are
only shown on hover, so they're still not the most discoverable.  It would be
even better to simply have clearer labels... but I think that might be tricky.)

You might also just update the histogram description to clarify what the
multiplied out event space is.  Suggested addition to the histogram description:

"""The password update bubble is shown when the user changes her password on a
website, say foo.com.  This histogram measures whether the user has any
passwords saved in the Chrome password manager for foo.com, and what action the
user performed on the offered bubble.  It's recorded when the bubble is
closed"""

[dvadym, 2015/08/21 14:09:53]

Thanks for such comprehensive answer. I've chosen the second option to write
down histogram description in more details:
The password submission event happened when the user changes her password
    on a website, say foo.com or types new password on a sign-in form for saved
    credential (which might mean that the password was changed in a different
    browser). The later case called password overriding. This histogram measures
    whether the user has any passwords saved in the Chrome password manager for
    foo.com or the password was overriden on sign-in form, and what action the
    user performed on the offered bubble. It's recorded when the bubble is
    closed.

I hope this explains what this metrics was introduced for.

+  <int value="1" label="NO_ACCOUNTS_CLICKED_NOPE"/>
+  <int value="2" label="NO_ACCOUNTS_NO_INTERACTION"/>
+  <int value="3" label="ONE_ACCOUNT_CLICKED_UPDATE"/>
+  <int value="4" label="ONE_ACCOUNT_CLICKED_NOPE"/>
+  <int value="5" label="ONE_ACCOUNT_NO_INTERACTION"/>
+  <int value="6" label="MULTIPLE_ACCOUNTS_CLICKED_UPDATE"/>
+  <int value="7" label="MULTIPLE_ACCOUNTS_CLICKED_NOPE"/>
+  <int value="8" label="MULTIPLE_ACCOUNTS_NO_INTERACTION"/>
+  <int value="9" label="PASSWORD_OVERRIDEN_CLICKED_UPDATE"/>
+  <int value="10" label="PASSWORD_OVERRIDEN_CLICKED_NOPE"/>
+  <int value="11" label="PASSWORD_OVERRIDEN_NO_INTERACTION"/>

[Ilya Sherman, 2015/08/20 00:30:51]

nit: "riden" -> "ridden"

[dvadym, 2015/08/20 11:10:11]

Done.

+</enum>
+
 <enum name="UpdatePolicy" type="int">
   <int value="0" label="UPDATES_DISABLED"/>
   <int value="1" label="AUTOMATIC_UPDATES"/>