WebUI cr-profile-avatar-selector: Fix focus background color.

Removing the --paper-button CSS mixin at r551209 changed the specificty of CSS
rules being applied. Fixed by using the keyboard-focus class directly, instead
of relying on yet another mixin --paper-button-flat-keyboard-focus.

Bug: 834739
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Iabd8f698a2b1ed65bb98af6a86fe2f957280ae61
Reviewed-on: https://chromium-review.googlesource.com/1019880Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552197}

ui/webui/resources/cr_elements/cr_profile_avatar_selector/cr_profile_avatar_selector.html

@@ -16,9 +16,6 @@
       }
 
       #avatar-grid .avatar {
-        --paper-button-flat-keyboard-focus: {
-          background: var(--paper-grey-400);
-        };
         background-color: var(--paper-grey-300);
         background-position: center;
         background-repeat: no-repeat;
@@ -34,6 +31,10 @@
         @apply --avatar-selector-avatar;
       }
 
+      #avatar-grid .avatar.keyboard-focus {
+        background-color: var(--paper-grey-400);
+      }
+
       #avatar-grid .avatar.iron-selected {
         border: 2px solid var(--google-blue-500);
       }