Chrome Browser Version: 52.0.2743.85/116 (Official Build)(64-bit) stable-channel peppy
OS: Chrome os
Chrome OS Version: 8350.60.0

Background:
We have used chromevox api(s)(https://cs.chromium.org/chromium/src/chrome/browser/resources/chromeos/chromevox/chromevox/injected/api.js)
to verbalize custom messages in our app. The chromevox was working as expected until chrome browser version 51.
After upgrading to version 52 the chromevox verbalization is broken on chrome os however it is working as expected
on desktop browsers(Window, Mac, Linux).

What steps will reproduce the problem?

1. Html file with following structure created with Polymer 1.0 >> Enable Chromevox

<custom-toolbar>
	#shadow-root
		<custom-menubar role='menubar' tabindex='0'>
			#shadow-root(open) 
			//--> [Please note: The below code is out from shadow-root.Which is replaced
			// with the <content> tag present inside shadow-root.]

			<custom-menu1 role='menu' aria-label='menu1' tabindex='0'>
				#shadow-root(open) //--> Please see above note
				<custom-menuitem1 role='menuitem' arialabel='menuitem1' tabindex='0'>
				</custom-menuitem1>
				....
			</custom-menu1>
			...
			<custom-menu10 role='menu' aria-label='submenu10'></custom-menu10>
		</custom-menubar>
</custom-toolbar>

[Note: Here, custom-toolbar, custom-menubar, custom-menu1 etc. are all custom elements.]

2. Navigate using tab key through main toolbar to Focus menu bar (custom-menubar)

[Please note: We listen to onFocus event and use cvox's speak api to verbalize our custom message.]

Expected: Chromevox  should verbalize the custom message along with aria attributes. 

Actual Result: It only verbalizes the aria-attributes and not the custom message. 

[Observations with chrome os version 52:]
1. With earlier browser versions(till 51), the chromevox's orange border does not appear for each item in our toolbar(because of chromevox
   issue with shadow-root). After version 52, the chromevox's orange border appears for each item in the toolbar. However, in desktop
   browsers it is still working as it was in version 51.
2. The code for calling cvox's speak api is getting executed with expected parameters.

Note: Chromevox verbalization is working as expected on desktop browser with chromevox extension (version - 53.0.2784.0).