cros: Fix user list background for a small number of users

Caused by uninitialized memory so the color would be random.

Change-Id: I49a306a426d723f2f75283b973c1b91e105f12c5
Reviewed-on: https://chromium-review.googlesource.com/1178896Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583854}

ash/login/ui/scrollable_users_list_view.h

@@ -81,11 +81,11 @@ class ASH_EXPORT ScrollableUsersListView : public views::ScrollView,
     static GradientParams BuildForStyle(LoginDisplayStyle style);
 
     // Start color for drawing linear gradient.
-    SkColor color_from;
+    SkColor color_from = SK_ColorTRANSPARENT;
     // End color for drawing linear gradient.
-    SkColor color_to;
+    SkColor color_to = SK_ColorTRANSPARENT;
     // Height of linear gradient.
-    SkScalar height;
+    SkScalar height = 0;
   };
 
   // Updates visibility of scroll bar thumb. Called when hover state changes.