Fix Polymer config for Video Player

In Video Player polymer_config.js was writing to console.error:
polymer_config.js:15 Polymer is already defined.

This was caused by <paper-slider.html> being included before any of
WebUI or Files app Polymer element had a chance to initialize with the
right config (polymer.html), which in turn was causing Polymer elements
to render without shadowRoot.

Change <files_icon_button> to include before <paper-slider> so it loads
the right config/polymer.html.

Test: --gtest_filter='VideoPlayerBrowserTest.CheckInitialElements' also manually checked for no error message and play button has shadowRoot.
Bug: 875525
Change-Id: I9888992cce762cd6eae8c31a8042119506fd5590
Reviewed-on: https://chromium-review.googlesource.com/1248342
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594620}

ui/file_manager/file_manager/background/js/test_util_base.js

@@ -32,6 +32,7 @@ function extractElementInfo(element, contentWindow, opt_styleNames) {
     // The hidden attribute is not in the element.attributes even if
     // element.hasAttribute('hidden') is true.
     hidden: !!element.hidden,
+    hasShadowRoot: !!element.shadowRoot
   };
 
   const styleNames = opt_styleNames || [];

ui/file_manager/integration_tests/video_player/check_elements.js

@@ -16,8 +16,13 @@ testcase.checkInitialElements = function() {
       return Promise.all([
         remoteCallVideoPlayer.waitForElement(appId, 'html[i18n-processed]'),
         remoteCallVideoPlayer.waitForElement(appId, 'div#video-player'),
-        remoteCallVideoPlayer.waitForElement(
-            appId, '#video-container > video'),
+        remoteCallVideoPlayer.waitForElement(appId, '#video-container > video'),
+        remoteCallVideoPlayer.waitForElement(appId, 'files-icon-button.play')
+            .then(function(element) {
+              // files-icon-button is a Polymer element and should have a
+              // shadowRoot.
+              chrome.test.assertTrue(element.hasShadowRoot);
+            }),
       ]);
     });
 };

ui/file_manager/video_player/video_player.html

@@ -15,8 +15,8 @@
   <link rel="stylesheet" type="text/css" href="css/arrow_box.css">
   <link rel="stylesheet" type="text/css" href="css/cast_menu.css">
 
-  <link rel="import" href="chrome://resources/polymer/v1_0/paper-slider/paper-slider.html">
   <link rel="import" href="chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/elements/files_icon_button.html">
+  <link rel="import" href="chrome://resources/polymer/v1_0/paper-slider/paper-slider.html">
 
   <style is="custom-style">
     files-icon-button {