This is a part of refactoring towards making the paintable tree
independent of the layout tree. Now, instead of transferring text
fragments from the layout tree to the paintable tree during the layout
commit phase, we allocate separate PaintableFragments that contain only
the information necessary for painting. Doing this also allows us to
get rid LineBoxes, as they are used only during layout.
This patch makes a few changes to the way we calculate line-height:
- `line-height: normal` is now resolved using metrics from the used
font (specifically, round(A + D + lineGap)).
- `line-height: calc(...)` is now resolved at style compute time.
- `line-height` values are now absolutized at style compute time.
As a consequence of the above, we no longer need to walk the DOM
ancestor chain looking for line-heights during style computation.
Instead, values are inherited, resolved and absolutized locally.
This is not only much faster, but also makes our line-height metrics
match those of other engines like Gecko and Blink.
If an element is created from JS, it might have its contents modified
before it is inserted into the document. In this case, we don't have a
shadow tree yet and so trying to set m_text_node's text content would
cause a null dereference. So let's not do that. :^)
That case also means that by the time we do create the shadow tree, we
have the text content already, so we can set it there.
Added a test to verify that we don't crash, and that the text content
appears in the textarea whether it was inserted by JS or by the HTML
parser.
