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

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 94024562c2ec8501b5c5aac4e601e6a04c2ab678..f84a4d36adc2a189ff08a59b5079eb562aeac873 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9871,6 +9871,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Cryptohome.MigrationUI.MigrationResult"
+    enum="MigrationUIMigrationResult">
+  <owner>fukino@chromium.org</owner>
+  <summary>
+    The result of encryption migration from eCryptfs to Ext4 dircrypto. The
+    recorded result is what the migration UI in Chrome side is notified from
+    cryptohomed.
+  </summary>
+</histogram>
+
+<histogram name="Cryptohome.MigrationUI.RemoveCryptohomeResult"
+    enum="MigrationUIRemoveCryptohomeResult">
+  <owner>fukino@chromium.org</owner>
+  <summary>
+    The result of the removal of user's cryptohome. When the migration UI is
+    notified that the migration failed, the UI tries to remove the user's
+    cryptohome to make sure that the user can create clean crytohome directory
+    in the next sign-in.
+  </summary>
+</histogram>
+
 <histogram name="Cryptohome.MigrationUI.UserChoice"
     enum="MigrationUIUserChoice">
   <owner>fukino@chromium.org</owner>
@@ -9881,6 +9902,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Cryptohome.MigrationUI.VisibleScreen" enum="MigrationUIScreen">
+  <owner>fukino@chromium.org</owner>
+  <summary>
+    How many times each screen in migration UI is shown to the user. A screen is
+    recorded as a visible screen when the screen is kept displayed at least for
+    a second.
+  </summary>
+</histogram>
+
 <histogram name="Cryptohome.TimeToCompleteDircryptoMigration" units="ms">
   <owner>dspaid@chromium.org</owner>
   <summary>