UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2866.0 Safari/537.36

Steps to reproduce the problem:
1. Load any extension, and navigate to a page where that extension injects content scripts to create a separate execution context. 
2. Open dev tools and from the execution context drop down navigate to the execution context for the extension.
3. In the console panel type

self.customElements

4. Observe that the reported value is null

5. From the drop down return to the page's execution context and type the same as above and observe that customElements reports CustomElementRegistry object.

What is the expected behavior?
Custom Elements v1 enabled by default works to be accessible in Extension contexts as well. 

What went wrong?
Custom Elements v1 is not currently loaded in Extension Context.

WebStore page: 

Did this work before? N/A 

Chrome version: 55.0.2866.0  Channel: canary
OS Version: 10.0
Flash Version: Shockwave Flash 23.0 r0

Is this intended behaviour? 
Custom Elements are particularly useful in Extensions that innject conntent scripts and UI elements into the page because shadowRoot can preserve author styles from cascade by embedder styles. Obviously shadowRoot is not CE v1, however, since they are all part of Web Commponents and CE v1 has landed, it works to be able to use the new v1 syntax and API in Web Components code in both web and Extension. Does it not?