[Editing][CodeHealth] assert_selection test doesn't need to have element.

third_party/WebKit/LayoutTests/editing/assert_selection.js

Index: third_party/WebKit/LayoutTests/editing/assert_selection.js
diff --git a/third_party/WebKit/LayoutTests/editing/assert_selection.js b/third_party/WebKit/LayoutTests/editing/assert_selection.js
index 014aa440c7c3dc8e39fc9e21bcedfe3cafed1dc6..14c29d63602ee59691175060bf278a9bb59b7169 100644
--- a/third_party/WebKit/LayoutTests/editing/assert_selection.js
+++ b/third_party/WebKit/LayoutTests/editing/assert_selection.js
@@ -509,6 +509,7 @@ class Sample {
   constructor(sampleText) {
     /** @const @type {!HTMLIFame} */
     this.iframe_ = document.createElement('iframe');
+    document.body = document.createElement("body");

[yosin_UTC9, 2016/07/07 05:55:48]

Let's check whether |document| has BODY or not to avoid to destruct existing
body.
if (!document.body)
  document.body = document.createElement("body");

[yoichio, 2016/07/07 06:42:07]

Done.

     document.body.appendChild(this.iframe_);
     /** @const @type {!HTMLDocument} */
     this.document_ = this.iframe_.contentDocument;