Get rid of Position::computedStyle

Get rid of Position::computedStyle
https://bugs.webkit.org/show_bug.cgi?id=117024

Reviewed by Andreas Kling.

Removed the function.

* dom/Position.cpp:
* dom/Position.h:
* editing/ApplyStyleCommand.cpp:
(WebCore::ApplyStyleCommand::positionToComputeInlineStyleChange): We need a position
inside the dummy element since we'll get the containing element's style instead
if we had the position before the dummy element.
* editing/EditingStyle.cpp:
(WebCore::StyleChange::StyleChange):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@150954 268f45cc-cd09-0410-ab3c-d52691b4dbfc