`BrowsingContext::m_parent` has been removed from the spec,
and previously `m_parent` was always null.
`BrowsingContext::is_top_level` was already always returning
true before because of that, and the updated spec algorithm
causes assertions to fail.
This fixes the following example:
```html
<a href="about:blank" target="test">a
<iframe name="test">
```
clicking the link twice no longer causes it to open in a new tab.
(cherry picked from commit e6a668ad913f0755840954c188c255fc5fc66606)
Previously, navigating to or from `about:newtab` caused a crash due to
inadvertent null dereferences when checking whether a request or
response to a request should be blocked as mixed content.
(cherry picked from commit 572ebe00eacd5aaeecc17207c75c6bf2327a3897)
