Use HashMap<OwnPtr> in RenderSVGResourceGradient

Use HashMap<OwnPtr> in RenderSVGResourceGradient
https://bugs.webkit.org/show_bug.cgi?id=75364

Reviewed by Daniel Bates.

* rendering/svg/RenderSVGResourceGradient.cpp:
(WebCore::RenderSVGResourceGradient::removeAllClientsFromCache):
(WebCore::RenderSVGResourceGradient::removeClientFromCache):
(WebCore::RenderSVGResourceGradient::applyResource): avoid looking
up the hash twice by using HashMap::add().
* rendering/svg/RenderSVGResourceGradient.h: rename m_gradient to m_gradientMap.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@103905 268f45cc-cd09-0410-ab3c-d52691b4dbfc