Extract colors from wallpaper and dynamically update login screen overlay

ui/login/account_picker/md_screen_account_picker.js

Index: ui/login/account_picker/md_screen_account_picker.js
diff --git a/ui/login/account_picker/md_screen_account_picker.js b/ui/login/account_picker/md_screen_account_picker.js
index 88ca6736d29011c8e2b851dff02530fac037c0be..56420401f6902aa066acbccdca38e46f0812538b 100644
--- a/ui/login/account_picker/md_screen_account_picker.js
+++ b/ui/login/account_picker/md_screen_account_picker.js
@@ -44,6 +44,7 @@ login.createScreen('AccountPickerScreen', 'account-picker', function() {
       'setPublicSessionLocales',
       'setPublicSessionKeyboardLayouts',
       'setLockScreenAppsState',
+      'setOverlayColors',
     ],
 
     preferredWidth_: 0,
@@ -81,6 +82,17 @@ login.createScreen('AccountPickerScreen', 'account-picker', function() {
       this.preferredHeight_ = height;
     },
 
+    /**
+      * Sets login screen overlay colors based on colors extracted from the
+      * wallpaper.
+      * @param {string} maskColor Color for the gradient mask.
+      * @param {string} scrollColor Color for the small pods container.
+      * @param {string} backgroundColor Color for the whole background.
+      */
+     setOverlayColors: function(maskColor, scrollColor, backgroundColor) {
+      $('pod-row').setOverlayColors(maskColor, scrollColor, backgroundColor);
+     },
+
     /**
      * When the account picker is being used to lock the screen, pressing the
      * exit accelerator key will sign out the active user as it would when