[Editing] Add null check in |DeleteSelectionCommand::removeRedundantBlocks()|

third_party/WebKit/LayoutTests/editing/execCommand/justify-center-editable-style-on-child-crash.html

+<title>Justify center and remove redounded div wont crash</title>

[Xiaocheng, 2016/12/22 03:25:51]

Could you mentioned the bug number in the layout test?

[chongz, 2016/12/22 05:14:50]

Done.

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<div><div>
+    <div></div>
+    <ul>
+        <li>
+            <div></div>
+            <input>
+            <style> * {-webkit-user-modify: read-write;}</style><div></div>
+        </li>
+    </ul></div></div>
+<script>
+test(() => {
+    const li = document.getElementsByTagName("li")[0];

[dominicc (has gone to gerrit), 2016/12/22 03:56:32]

If you just do querySelector you'll get the 0th one.

[chongz, 2016/12/22 05:14:50]

Done.

+    window.getSelection().collapse(li, 0);
+    document.execCommand('JustifyRight');
+}, 'Justify center and remove redounded div wont crash');

[dominicc (has gone to gerrit), 2016/12/22 03:56:32]

This description needs some work. Is this Justify, center and remove something?
If it's a sequence a comma makes it easier to understand that.

What is redounded?

wont => won't

[chongz, 2016/12/22 05:14:50]

Sorry for the typo... I've added a comment for the redundant <div> and moved
description to <title> (Since there is only one test case).

+</script>