This implements the idea of automatically setting a content proc's
render root based on the render root enclosing the iframe that
points to it. There was a bit of cleanup in here that was a bit
tricky to extract from the core patch revolving around how we
use the Api(...) helper. This was to avoid the situation where
we use the Api(...) helper before our render root is initialized,
when we don't actually have to. I.e., when we just want the root
WebRenderAPI in all cases.
An alternative to this approach could be to fully built out the
WebRender transactions and just queue those up to be sent. However,
transaction building has various side effects which are committed
before the transaction is actually sent, so we would have to build
out some scheme for deferring those as well. This seemed simpler.
Patch primarily written by :dthayer
Differential Revision: https://phabricator.services.mozilla.com/D37078
227074a12b
