Eliminate styleDidChange with StyleDifferenceEqual when updates are actually necessary

Eliminate styleDidChange with StyleDifferenceEqual when updates are actually necessary
https://bugs.webkit.org/show_bug.cgi?id=144198

Followup: fix assertions seen in fullscreen and pseudo-element tests. These code
paths set the style to the existing pointer, but with a SyntheticStyleChange.
We have to avoid an early return in this case.

* rendering/RenderElement.cpp:
(WebCore::RenderElement::setStyle):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183461 268f45cc-cd09-0410-ab3c-d52691b4dbfc