[Webkit] Span elements created while joining adjacent elements

[Olek Nowodziński]

It's a follow-up of #9998, which covers a single ​case.

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Example 1

1. Set data

   <p>foo</p>
   <p>^bar</p>
   

2. Backspace.
3. <p>foo<span style="line-height:1.6">bar</span></p>

Example 2

1. Set data

   <h1>foo</h1>
   <p>^bar</p>
   

2. Backspace.
3. <h1>foo<span style="font-size:13px; line-height:1.6">bar</span></h1>