DevTools: Fix an exception on adding watch expression.

third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 1268 matching lines @@
     }
 
     if (!this.editable || this._isSASSStyle())
       return;
 
     var config = new UI.InplaceEditor.Config(
         this._editingMediaCommitted.bind(this, media), this._editingMediaCancelled.bind(this, element), undefined,
         this._editingMediaBlurHandler.bind(this));
     UI.InplaceEditor.startEditing(element, config);
 
-    element.getComponentSelection().setBaseAndExtent(element, 0, element, 1);
+    element.getComponentSelection().selectAllChildren(element);
     this._parentPane.setEditingStyle(true);
     var parentMediaElement = element.enclosingNodeOrSelfWithClass('media');
     parentMediaElement.classList.add('editing-media');
 
     event.consume(true);
   }
 
   /**
    * @param {!Element} element
    */
@@ skipping 110 matching lines @@
     if (UI.isBeingEdited(element))
       return;
 
     element.scrollIntoViewIfNeeded(false);
     element.textContent = element.textContent;  // Reset selector marks in group.
 
     var config =
         new UI.InplaceEditor.Config(this.editingSelectorCommitted.bind(this), this.editingSelectorCancelled.bind(this));
     UI.InplaceEditor.startEditing(this._selectorElement, config);
 
-    element.getComponentSelection().setBaseAndExtent(element, 0, element, 1);
+    element.getComponentSelection().selectAllChildren(element);
     this._parentPane.setEditingStyle(true);
     if (element.classList.contains('simple-selector'))
       this._navigateToSelectorSource(0, false);
   }
 
   /**
    * @param {string} moveDirection
    */
   _moveEditorFromSelector(moveDirection) {
     this._markSelectorMatches();
@@ skipping 931 matching lines @@
       this._prompt.on(UI.TextPrompt.TextChangedEvent, this._applyFreeFlowStyleTextEdit.bind(this));
 
     var proxyElement = this._prompt.attachAndStartEditing(selectElement, blurListener.bind(this, context));
     this._navigateToSource(selectElement, true);
 
     proxyElement.addEventListener('keydown', this._editingNameValueKeyDown.bind(this, context), false);
     proxyElement.addEventListener('keypress', this._editingNameValueKeyPress.bind(this, context), false);
     if (isEditingName)
       proxyElement.addEventListener('paste', pasteHandler.bind(this, context), false);
 
-    selectElement.getComponentSelection().setBaseAndExtent(
-        selectElement, 0, selectElement, selectElement.firstChild ? 1 : 0);
+    selectElement.getComponentSelection().selectAllChildren(selectElement);
   }
 
   /**
    * @param {!Elements.StylePropertyTreeElement.Context} context
    * @param {!Event} event
    */
   _editingNameValueKeyDown(context, event) {
     if (event.handled)
       return;
 
@@ skipping 697 matching lines @@
   }
 
   /**
    * @override
    * @return {!UI.ToolbarItem}
    */
   item() {
     return this._button;
   }
 };