Chrome Version       : 63.0.3239.140 (ChromeVox Next)
OS Version           : Chrome OS
URLs (if applicable) : 
Other browsers tested:
  Add OK or FAIL after other browsers where you have tested this issue:
     Safari: N/A
    Firefox: N/A
    IE/Edge: N/A

What steps will reproduce the problem?
1. Enable ChromeVox on a Chrome OS device
2. Visit https://codepen.io/anon/pen/XZBmBw
3. Turn on sticky mode (hit the search button twice)
4. Press 'h' a few times to iterate through headings
5. When you reach "We all live in a yellow submarine", notice that only the first part is voiced

What is the expected result?
The whole heading ("We all live in a yellow submarine") should be voiced.

What happens instead of that?
Only the portion ("We all live in a") before the inner tag is voiced.

Please provide any additional information below. Attach a screenshot if
possible.
As you can see in this code example (https://codepen.io/anon/pen/XZBmBw) the issue occurs with a simple `<span>` inside of the heading. This should be valid.

UserAgentString: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.167 Safari/537.36