Allow DoubleClick ads to explicitly request an ad size.

[cramforce]

The default continues to be to infer the size from the size of the ad slot.

Closes #1429

[ampsauce]

Tests failed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: BUSTED

Commit: c067a4f
Build details: https://travis-ci.org/ampsauce/amphtml/builds/102413770

(Please note that this is a fully automated comment.)

[ampsauce]

Tests failed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: BUSTED

Commit: ac3a52f597e88053eb2bf70164a4a7a8334d6891
Build details: https://travis-ci.org/ampsauce/amphtml/builds/102413392

(Please note that this is a fully automated comment.)

[jridgewell]

LGTM, but I'm not sure what the utility in this is. It makes a big amp-ad block with smaller/larger content inside it?

[cramforce]

@jridgewell This is for a special case where magic dimensions are used (like 3x3) to e.g. get ads that auto size to the container dimension.