Fix selection{Start,End} when selectionDirection is "backward"

Per the spec, selectionStart and selectionEnd should return the same
values regardless of the selectionDirection. (That is, selectionStart is
always less than or equal to selectionEnd; the direction then implies
which of selectionStart or selectionEnd is the cursor position.)

There was no explicit WPT test for this, so I added one.

This bug was initially quite hard to wrap my head around, and I think
part of the problem is the code in TextInput. Therefore, in the process
of fixing it I have refactored the implementation of TextInput:

* Rename selection_begin to selection_origin. This value doesn't
  necessarily correspond directly to the selectionStart DOM value - in
  the case of a backward selection, it corresponds to selectionEnd.
  I feel that "origin" doesn't imply a specific ordering as strongly as
  "begin" (or "start" for that matter) does.

* In various other cases where "begin" is used as a synonym for "start",
  just use "start" for consistency.

* Implement selection_start() and selection_end() methods (and their
  _offset() variants) which directly correspond to their DOM
  equivalents.

* Rename other related methods to make them less wordy and more
  consistent / intention-revealing.

* Add assertions to assert_ok_selection() to ensure that our assumptions
  about the ordering of selection_origin and edit_point are met. This
  then revealed a bug in adjust_selection_for_horizontal_change() where
  the value of selection_direction was not maintained correctly (causing
  a unit test failure when the new assertion failed).

components/script/dom/textcontrol.rs

@@ -123,11 +123,11 @@ pub trait TextControl: DerivedFrom<EventTarget> + DerivedFrom<Node> {
     }
 
     fn selection_start(&self) -> u32 {
-        self.textinput().borrow().get_selection_start()
+        self.textinput().borrow().selection_start_offset() as u32
     }
 
     fn selection_end(&self) -> u32 {
-        self.textinput().borrow().get_absolute_insertion_point() as u32
+        self.textinput().borrow().selection_end_offset() as u32
     }
 
     fn selection_direction(&self) -> SelectionDirection {