Makes assert_selection() to set focus to sample IFRAME

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 4c4ead925d78b20cba4ebe74a42004950c36632f..6597ccb9bdc69671d74dad0fe6494a8c07ff847f 100644
--- a/third_party/WebKit/LayoutTests/editing/assert_selection.js
+++ b/third_party/WebKit/LayoutTests/editing/assert_selection.js
@@ -554,6 +554,9 @@ class Sample {
     this.selection_.document.offsetTop = this.iframe_.offsetTop;
     this.selection_.setClipboardData = setClipboardData;
 
+    // Set focus to sample IFRAME to make |eventSender| and
+    // |testRunner.execCommand()| to work on sample rather than main frame.
+    this.iframe_.focus();
     this.load(sampleText);
   }