cros: Add UMA metrics in migration UI to get more information from migration failures.

tools/metrics/actions/actions.xml

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is a comprehensive list of Chrome user actions along with the owner
 and description for each user action.
 
@@ skipping 3703 matching lines @@
 <action name="CriticalNotification_Ignore">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
 </action>
 
 <action name="CriticalNotification_Restart">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
 </action>
 
+<action name="Cryptohome.MigrationUI.RestartClickedOnFailure">

[Ilya Sherman, 2017/05/31 23:34:44]

If you're just interested in understanding how often users click to restart
after seeing a failure screen, I'd recommend using an enumerated histogram. 
OTOH, if you want to analyze sequences of user actions that include this event,
then user actions seem reasonable.

[fukino, 2017/06/01 09:28:38]

We are not going to analyze sequences for both
Cryptohome.MigrationUI.RestartClickedOnFailure and
Cryptohome.MigrationUI.ShowFailureScreen.
It seems I don't need to add user actions in this CL.
I updated the CL as follows:

* Cryptohome.MigrationUI.RestartClickedOnFailure
I merged this event to Cryptohome.MigrationUI.UserChoice histogram. They enums
for UserChoice correspond to click events on buttons, so we can naturally add
this event ("Restart" clicked on failure screen) to UserChoice events.

* Cryptohome.MigrationUI.ShowFailureScreen
I added a new Cryptohome.MigrationUI.VisibleScreen histogram, make it more
general.
I record every visible screen (i.e. screen which is shown at least 1 second),
and it is not limited to the failure screen.

+  <owner>fukino@chromium.org</owner>
+  <description>
+    Restart button on the Ext4 migration failure screen is clicked.
+  </description>
+</action>
+
+<action name="Cryptohome.MigrationUI.ShowFailureScreen">

[fukino, 2017/05/31 12:21:28]

Based on "Generally, Do Not Emit Impressions" section in
https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/actio...,
I thought it was OK (not good, though) as it is attention-grabbing.
Do I understand correctly? (Is there a better option?)

[Ilya Sherman, 2017/05/31 23:34:44]

Optional nit: I'd write "Showed" instead of "Show".

+  <owner>fukino@chromium.org</owner>
+  <description>
+    The migration failure screen is shown after the user attempts to do the
+    encryption migration.
+  </description>
+</action>
+
 <action name="Cryptohome.PKCS11InitFail">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
   <obsolete>Deprecated as of 11/2015.</obsolete>
 </action>
 
 <action name="CustomTabs.CloseButtonClicked">
   <owner>dfalcantara@chromium.org</owner>
   <description>Close button of the CustomTabActivity was clicked.</description>
 </action>
@@ skipping 13703 matching lines @@
 <action name="webapps.AddShortcut.Manifest">
   <owner>mlamouri@chromium.org</owner>
   <description>
     Shortcut added to the homescreen with a valid Manifest. This action will be
     recorded in addition to AppShortcut, AppShortcutApple and Bookmark actions
     to keep a trend of whether the webapp also add meta tags.
   </description>
 </action>
 
 </actions>