Wallpaper: Add shadow to wallpaper manager

Remove the alphaEnabled flag which created a flat transparent window.

BUG=936235
TEST=run on eve and verified that shadows appear

Change-Id: Ia2a10654cdacbb0c75991513984e3ec8652d2c13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755009
Auto-Submit: May Lippert <maybelle@chromium.org>
Commit-Queue: May Lippert <maybelle@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687110}

chrome/browser/resources/chromeos/wallpaper_manager/js/event_page.js

@@ -196,8 +196,7 @@ chrome.app.runtime.onLaunched.addListener(function() {
   var options = {
     frame: 'none',
     innerBounds: {width: 768, height: 512, minWidth: 768, minHeight: 512},
-    resizable: true,
-    alphaEnabled: true
+    resizable: true
   };
 
   chrome.app.window.create('main.html', options, function(window) {