Add support for SlimCut Media ads in amp-ad

[m1ck43l]

Hello,

I've followed the guidelines and examples to add support for SlimCut Media ads into amp-ad.

• No error with gulp lint
• Ran & passed tests "test-ads-config.js,test-integration.js"
• Tested locally and on heroku (http://ampscm.herokuapp.com/examples/ads.amp.max.html?type=slimcutmedia)
• Signed Google CLA

Please review this pull request, thanks!

Implements #7254

[jridgewell]

/to @lannka

[lannka]

The example does not load an ad locally. could you check?

[m1ck43l]

Example was limited to the heroku test site, sorry for that.
The example is now running locally.

We authorize domains localhost and ampscm.herokuapp.com.

Thanks

[lannka]

@m1ck43l the example works now. But I see a noticeable blank time between the loading indicator and the ad display. Could you confirm you're calling render-start API at the right time?

[m1ck43l]

@lannka We plan to add a loading screen to avoid the blank time. Do you need it to be implemented right for the PR? Thanks

[m1ck43l]

@lannka We've added the loading screen. The white screen will disappear once we have the ad. Thanks

[lannka]

@m1ck43l sorry for the late reply. AMP runtime already provided a loading indicator. Are you able to call render-start at the time you hide your loading screen? If not, I'd prefer no 2nd loading indicator.

[m1ck43l]

@lannka We've removed the second loading indicator. Should be good now.