[Editing][CodeHealth] Allow assert_selection have no anchor sample text.

This enable us to test creating new selection.

TEST=LayoutTests/editing/assert_selection.html

Review-Url: https://codereview.chromium.org/2114903002
Cr-Commit-Position: refs/heads/master@{#403433}

third_party/WebKit/LayoutTests/editing/assert_selection.html

@@ -14,6 +14,7 @@ function checked_assert_selection(input, command, output) {
 }
 
 test(() => {
+    assert_selection('foo', 'noop', 'foo');
     assert_selection('|foo', 'noop', '|foo');
     assert_selection('f|oo', 'noop', 'f|oo');
     assert_selection('foo|', 'noop', 'foo|');
@@ -60,11 +61,6 @@ test(() => {
         '|<table><tbody><tr><td>foo</td></tr></tbody></table>^');
 }, 'markers around table');
 
-test(() => {
-    assert_equals(checked_assert_selection('foo', 'noop', 'baz'),
-        'You should specify caret position in "foo".');
-}, 'no marker in input');
-
 test(() => {
     assert_equals(checked_assert_selection('fo|o', 'noop', 'fo|o'),
         'no exception');

third_party/WebKit/LayoutTests/editing/assert_selection.js

@@ -462,8 +462,6 @@ class Serializer {
    * @param {!HTMLDocument} document
    */
   serialize(document) {
-    if (this.selection_.isNone)
-      return document.body.firstChild.outerHTML;
     this.serializeChildren(document.body);
     return this.strings_.join('');
   }
@@ -534,7 +532,7 @@ class Sample {
   load(sampleText) {
     const anchorMarker = sampleText.indexOf('^');
     const focusMarker = sampleText.indexOf('|');
-    if (focusMarker < 0) {
+    if (focusMarker < 0 && anchorMarker >= 0) {
       throw new Error(`You should specify caret position in "${sampleText}".`);
     }
     if (focusMarker != sampleText.lastIndexOf('|')) {