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

tools/metrics/actions/actions.xml

Index: tools/metrics/actions/actions.xml
diff --git a/tools/metrics/actions/actions.xml b/tools/metrics/actions/actions.xml
index 3fc8d7851858d03f876f1af219e55c6a50650edf..17c244045364ecd2da4b7d5c445e7d4accc99807 100644
--- a/tools/metrics/actions/actions.xml
+++ b/tools/metrics/actions/actions.xml
@@ -3721,6 +3721,21 @@ should be able to be added at any place in this file.
   <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>