Improve creative resizing logic

[dgirardi]

When using native-render.js, after the ad is rendered, we take the ad height and send it to Prebid for it to be set on the frame's container:

prebid-universal-creative/src/nativeAssetManager.js

Line 381 in 0d0ef04

requestHeightResize(bid.adId, (document.body.clientHeight || document.body.offsetHeight), document.body.clientWidth);

This is because GPT will fix the safeframe's height based on the contents it knows about, and it does not know that we are dynamically generating it. However, this approach has problems (see #190 (comment)):

• at the time we requestHeightResize, we do not necessarily know the true height of the creative, because images and scripts may still need to be loaded. We should try to wait until page load is complete before requesting the resize.
• it is possible to tell GPT to allow height: auto safeframes (allowPushExpansion), in which case we should not fix the height ourselves.

[patmmccann]

allowPushExpansion support still needed