The code was trying to assert that we had frames constructed for all the nodes
in the parent chain, but we don't bail out in the
!GetContentInsertionFrameFor(aContainer) in the case that it's a children
element, because they actually have no insertion frame, though their children
do.

Move the LazyFC check after the insertion point check. That makes the previous
check work on the insertion point of the child, which makes it sound.

This also fixes bug 1410020, and with it a Shadow DOM test-case that was failing
because we had two sibling assigned to two different <slot>s, and the second one
wasn't getting properly flagged, and thus the second sibling never got a frame.

The other two test failures in this test are an event dispatch failure, where
the position of the target is not what the test expects (we don't account for
margin and padding). Filed that as bug 1450027.

Also, added a test for which we have wrong layout without these patches, and
that crashes with "Called Servo_Element_IsDisplayNone" with the first patch of
this bug applied but not this one, due to the bogus check mentioned above.
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1303605
gecko-commit: 12a824f8d55a8fb0396fb2132974f8223c6a9606
gecko-integration-branch: central
gecko-reviewers: bz