Bug 727125 - Lazily compute LineBaselineOffset when needed so it is present after a dynamic change of 'text-decoration'.  r=dbaron

This fixes the positioning of underlines set on a block or its ancestor
when drawn on children of a block that have a vertical-align !=
baseline.

The lazy computation is done all at once for all children of a block to
avoid O(N^2) searches for the line containing a frame.
Bug 727125 - Update overflow areas for text frames on text-decoration change to ensure text-decoration on hover is visible. r=dbaron