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 56a5e0f31264c79a0bd2467a8dde1e4bbd1da036..66390eaefebc17d79ae0a466d0f7f2ba822d0684 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9788,6 +9788,27 @@ 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>
@@ -9798,6 +9819,15 @@ 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 gets visible to the user. A

[Ilya Sherman, 2017/06/02 00:04:33]

nit: s/gets visible/is shown

[fukino, 2017/06/02 03:39:36]

Done.

+    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>