Deploy RenderPtr in RenderMathMLOperator::updateFromElement().

Deploy RenderPtr in RenderMathMLOperator::updateFromElement().
<https://webkit.org/b/126628>

Reviewed by Antti Koivisto.

* rendering/mathml/RenderMathMLOperator.cpp:
(WebCore::RenderMathMLOperator::updateFromElement):

    Use RenderPtr/createRenderer for renderer creation.
    Removed an unnecessary null check (text is always created.)
    Also use RenderStyle::createAnonymousStyleWithDisplay()
    helper to shorten down the code a bit.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161501 268f45cc-cd09-0410-ab3c-d52691b4dbfc