[ContentEditable] Wrong inline styles result when selection start with readonly element

[garry.yao]

1. Load the following HTML and apply the following selection, without putting cursor into the editor:

<p>Para [1</p>
<p contenteditable="false">Non] Editable</p>
<p>Para 2</p>

2. Apply 'Bold' command.

Current results:

<p>
	Para <strong>1</strong></p>
<p contenteditable="false">
	<span>Non Editable</span></p>
<p>
	<strong>Para 2</strong></p>

Expected results:

<p>
	Para <strong>1</strong></p>
<p contenteditable="false">
	<span>Non Editable</span></p>
<p>
	Para 2</p>

Reproduced on the ​contenteditable feature branch.

[garry.yao]

Changes are committed with [5605].