<amp-ad> Page is not showing any ads or fallbacks

[JamesIves]

Describe the bug
A clear and concise description of what the bug is.

The amp-ad example page is not rendering any advertisements. I see a blank screen.

Additionally when you open the amp-ad page in the playground the fallbacks are not rendering correctly despite the document being valid.

To Reproduce
Steps to reproduce the behavior:

1. Visit https://ampbyexample.com/playground/#url=https://ampbyexample.com/components/amp-ad/source/ and https://ampbyexample.com/components/amp-ad/
2. Observe the neither the ads, or the fallback render correctly.

Expected behavior
A clear and concise description of what you expected to happen.

The advertisement and the fallback should render.

Desktop (please complete the following information):

• OS:
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: Macbook Pro
• OS: Mac OSX 10.14.1
• Browser: Chrome
• Version: 71.0.3578.98 (Official Build) (64-bit)