Wait for the battery level before starting encryption migration.

chrome/browser/resources/chromeos/login/screen_encryption_migration.js

Index: chrome/browser/resources/chromeos/login/screen_encryption_migration.js
diff --git a/chrome/browser/resources/chromeos/login/screen_encryption_migration.js b/chrome/browser/resources/chromeos/login/screen_encryption_migration.js
index ca8cc6d0047dadbb62dc60181ea65a9527f27136..a177cf979fe643aa95d3d968dd00fdfa0e9470af 100644
--- a/chrome/browser/resources/chromeos/login/screen_encryption_migration.js
+++ b/chrome/browser/resources/chromeos/login/screen_encryption_migration.js
@@ -9,6 +9,8 @@ login.createScreen('EncryptionMigrationScreen', 'encryption-migration',
       'setUIState',
       'setMigrationProgress',
       'setIsResuming',
+      'setBatteryPercent',
+      'setIsMigrationAccepted',
     ],
 
     /**
@@ -67,5 +69,26 @@ login.createScreen('EncryptionMigrationScreen', 'encryption-migration',
     setIsResuming: function(isResuming) {
       $('encryption-migration-element').isResuming = isResuming;
     },
+
+    /**
+     * Updates battery level of the device.
+     * @param {number} batteryPercent Battery level in percent.
+     * @param {boolean} isEnoughBattery True if the battery is enough.
+     */
+    setBatteryPercent: function(batteryPercent, isEnoughBattery) {
+      $('encryption-migration-element').batteryPercent =
+          Math.floor(batteryPercent);
+      $('encryption-migration-element').isEnoughBattery = isEnoughBattery;
+    },
+
+    /**
+     * Updates whether the migration is already accepted. This information is
+     * used to disable the button to start migration.
+     * @param {boolean} isMigrationAccepted True if migration was accepted.
+     */
+    setIsMigrationAccepted: function(isMigrationAccepted) {
+      $('encryption-migration-element').isMigrationAccepted =
+          isMigrationAccepted;

[xiyuan, 2017/04/12 16:56:56]

nit: I would do 

  $('encryption-migration-element').isMigrationAccepted = true;

in 'upgrade' event handler around line 25 to avoid a round trip to C++.

[fukino, 2017/04/12 22:59:02]

Done.
I toggled isMigrationAccepted property inside <encryption-migration> element on
'upgrade' event handler.

+    },
   };
 });