RenderLayer: Store reflection renderer in a RenderPtr.

RenderLayer: Store reflection renderer in a RenderPtr.
<https://webkit.org/b/126273>

Reviewed by Anders Carlsson.

* rendering/RenderLayer.h:
* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::RenderLayer):
(WebCore::RenderLayer::calculateClipRects):

    Turn RenderLayer::m_reflection into a RenderPtr<RenderReplica>
    instead of a raw pointer.

* WebCore.xcodeproj/project.pbxproj:

    Add RenderPtr.h to private headers.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161123 268f45cc-cd09-0410-ab3c-d52691b4dbfc