Editor creates a `<br>` element to end of a block if last line
of the block is empty because caret should be placed as there is an empty
line. Such special `<br>` element has `type` attribute whose value is "_moz".
However, adding/removing the attribute is expensive and such hacky attribute
shouldn't be referred nor changed by web apps.
Therefore, this patch makes `HTMLBRElement` take another specific flag whether
it's a special node for empty last line. For making the meaning clearer,
this patch calls the such `<br>` elements as "padding `<br>` element for
empty last line" insead of "moz-br". So, this patch also includes a lot of
renaming methods and variables, and modifying related comments.
Note that with this change, `IMEContentObserver` counts the padding `<br>`
element in `<textarea>` because it's inserted before setting the new flag
and setting the flag does not cause DOM tree mutation. This issue will be
fixed by the following patches.
Differential Revision: https://phabricator.services.mozilla.com/D39858
--HG--
extra : moz-landing-system : lando
