Normally, assigning to e.g document.body.onload will forward to
window.onload. However, in a detached DOM tree, there is no associated
window, so we have nowhere to forward to, making this a no-op.
The bulk of this change is making Document::window() return a nullable
pointer, as documents created by DOMParser or DOMImplementation do not
have an associated window object, and so must be able to return null
from here.
The default canvas size is 300x150 pixels. If the element or document
we are trying to screenshot for the WebDriver is not at least that size,
then we will create a canvas that is wider or taller than the actual
element we are painting, resulting in a bunch of transparent pixels
falling off the end.
This fixes 14 WPT css/CSS2/floats tests that we run in CI, and
presumably a ton of other reftests in the WPT test suite.
This required dealing with a *lot* of fallout, but it's all basically
just switching from DeprecatedFlyString to either FlyString or
Optional<FlyString> in a hundred places to accommodate the change.
This will allow for implementing the screenshot action closer to the
spec, as we can now use HTMLCanvasElement to encode the bitmap, and
capture the screenshot on the animation frame loop.
