Reland of Make FrameSelection to hold non-canonicalized positions

third_party/WebKit/LayoutTests/editing/undo/redo-selection-modify-crash.html

 <script src="../../resources/testharness.js"></script>
 <script src="../../resources/testharnessreport.js"></script>
 <script src="../assert_selection.js"></script>
 <div id="log"></div>
 <script>
 test(() => {
     assert_selection(
         '<div contenteditable>|foo<br><i>bar</i></div>',
         selection => {
             // Make base/extent and start/end different.
             selection.modify('extend', 'forward', 'word');
             selection.document.execCommand('justifyLeft');
             selection.document.execCommand('undo');
             // Remove an anchor node for extent position of redo
             selection.document.querySelector('i').remove();
             selection.document.execCommand('redo');
         },
-        navigator.platform.indexOf('Mac') == 0
-            ? '<div contenteditable>^foo|<br></div>'
-            : '<div contenteditable>^foo<br>|</div>');
+        '<div contenteditable>^foo|<br></div>');
 });
 </script>