Use is<>() / downcast<>() for StyleImage

Use is<>() / downcast<>() for StyleImage
https://bugs.webkit.org/show_bug.cgi?id=137824

Reviewed by Andreas Kling.

Use is<>() / downcast<>() for StyleImage and clean up the surrouding
code.

No new tests, no behavior change.

* css/CSSCursorImageValue.cpp:
(WebCore::CSSCursorImageValue::detachPendingImage):
(WebCore::CSSCursorImageValue::cachedImage):
(WebCore::CSSCursorImageValue::cachedImageURL):
* css/CSSImageSetValue.cpp:
(WebCore::CSSImageSetValue::detachPendingImage):
(WebCore::CSSImageSetValue::~CSSImageSetValue):
(WebCore::CSSImageSetValue::cachedImageSet):
(WebCore::CSSImageSetValue::hasFailedOrCanceledSubresources):
* css/CSSImageValue.cpp:
(WebCore::CSSImageValue::detachPendingImage):
(WebCore::CSSImageValue::cachedImage):
(WebCore::CSSImageValue::hasFailedOrCanceledSubresources):
* css/StyleResolver.cpp:
(WebCore::StyleResolver::loadPendingShapeImage):
(WebCore::StyleResolver::loadPendingImages):
* page/PageSerializer.cpp:
(WebCore::PageSerializer::retrieveResourcesForProperties):
* page/animation/CSSPropertyAnimation.cpp:
(WebCore::blendFunc):
* rendering/style/StyleCachedImage.h:
* rendering/style/StyleCachedImageSet.h:
* rendering/style/StyleGeneratedImage.h:
* rendering/style/StyleImage.h:
* rendering/style/StylePendingImage.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@174836 268f45cc-cd09-0410-ab3c-d52691b4dbfc