BrowsingContext shouldn't be scrolling itself, instead it has to update
the layout (to ensure that we have current document metrics, and then
ask the PageClient nicely to scroll it.
This fixes an issue where BrowsingContext sometimes believed itself to
be scrolled, but OOPWV had a different idea.
This commit moves the regular handling of links to the anchor elements'
activation behavior, and implements a few auxiliary algorithms as
defined by the HTML specification.
Note that certain things such as javascript links, fragments and opening
a new tab are still handled directly in EventHandler, but they have been
moved to handle_mouseup so that it behaves closer to how it would if it
was entirely up-to-spec.
Right now the only functionality supported is getting/setting via JS
and resetting when browsing cross origin.
The HTML Specification (7.11 Browsing the web) also specifies how the
name should be restored from history entries, but we don't have those
yet.
A top level browsing context is a browsing context with no parent
browsing context.
However, we considered a top level browsing context to be a browsing
context with no associated browsing context container.
