[fileapp] Remove FilesMessage createElement_ member

Create the element content the way we do with the <bread-crumb> custom
element (which came latter).

No change in behavior: covered by FilesMessage unittest.

Bug: code-health
Change-Id: Id568aa6b5644a712db88ba5f4fa8d3d13f323b06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500685
Commit-Queue: Noel Gordon <noel@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821070}

ui/file_manager/file_manager/foreground/elements/files_message.js

@@ -74,8 +74,10 @@ const filesMessageTemplate = `
  */
 class FilesMessage extends HTMLElement {
   constructor() {
-    super();
-    this.createElement_();
+    /**
+     * Create element content.
+     */
+    super().attachShadow({mode: 'open'}).innerHTML = filesMessageTemplate;
 
     /**
      * FilesMessage visual signals user callback.
@@ -84,14 +86,6 @@ class FilesMessage extends HTMLElement {
     this.signal_ = console.log;
   }
 
-  /**
-   * Creates FilesMessage element.
-   * @private
-   */
-  createElement_() {
-    this.attachShadow({mode: 'open'}).innerHTML = filesMessageTemplate;
-  }
-
   /**
    * Sets |this| properties (even its HTMLElement properties) from an object.
    * @param {!Object} object Settable property name/value pairs.