Changing typing style with font size delta overrides the previous font size delta
https://bugs.webkit.org/show_bug.cgi?id=113923

Reviewed by Andreas Kling.

The bug was caused by a new font size delta value overriding the old one as
opposed to being added to the old value. Fixed that in overrideWithStyle.

Also made overrideWithStyle share code with mergeStyle since there is already
code to merge text-decorations there.

Unfortunately, adding a test for this fix proved to be hard because we don't
expose any API to directly set typing style.

* editing/EditingStyle.cpp:
(WebCore::EditingStyle::overrideWithStyle): Use mergeStyle.
(WebCore::EditingStyle::mergeStyle): Add font size deltas as opposed to simply
using the new value.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147661 268f45cc-cd09-0410-ab3c-d52691b4dbfc