Bug 1580565 - Part 2: Delay creating nsFrameLoader in static clone until embedder has window, r=smaug

Follow-up parts in this bug depend on being able to read the `nsGlobalWindow`
which embeds a `nsFrameLoader` within `CreateBrowsingContext`, which is called
from the `nsFrameLoader` constructor. Unfortunately, we depend on creating the
`nsFrameLoader` and `BrowsingContext` before we have the window as part of the
fix to bug 1577711.

This patch changes `BuildNestedPrintObjects` to instead use a list of pending
clones stored on the parent `Document` object, and delays creation of the
`nsFrameLoader`, and thus the inner `BrowsingContext`, until after the document
has an owner global.

Due to the low number of automated tests for printing, I manually tested
print-previewing both the reduced test case from bug 1577711, a wikipedia
article, and 'data:text/html,<object data="data:text/html,hi">' to avoid
regressions.

Differential Revision: https://phabricator.services.mozilla.com/D71236