SVGViewSpec objects should mark relevant SVG elements

SVGViewSpec objects should mark relevant SVG elements
https://bugs.webkit.org/show_bug.cgi?id=192567
<rdar://problem/46491325>

Reviewed by Ryosuke Niwa.

Source/WebCore:

SVGViewSpec elements reflect the state of an underlying SVGElement. Teach the mark algorithm to
recognize the relevant SVGElement as active as long as the SVGViewSpec is active.

Update SVGElement so that it can vend WeakPtrs. I also noticed that SVGAttributeOwner used a bare
pointer to the SVGElement, so switched to a WeakPtr.

Test: svg/animations/view-dependency-crash.html

* Sources.txt: Add new files.
* WebCore.xcodeproj/project.pbxproj: Ditto.
* bindings/js/JSSVGViewSpecCustom.cpp: Added.
(WebCore::JSSVGViewSpec::visitAdditionalChildren):
* svg/SVGElement.h:
* svg/SVGPathElement.h:
* svg/SVGViewSpec.cpp:
(WebCore::SVGViewSpec::SVGViewSpec): Hold a weak pointer (rather than a bare pointer) to the underlying element.
* svg/SVGViewSpec.h:
* svg/SVGViewSpec.idl:
* svg/properties/SVGAttributeOwnerProxy.cpp: Added.
(WebCore::SVGAttributeOwnerProxy::SVGAttributeOwnerProxy): Hold a weak pointer (rather than a bare pointer) to
the underling SVGElement.
(WebCore::SVGAttributeOwnerProxy::element const): Ditto.
* svg/properties/SVGAttributeOwnerProxy.h:
(WebCore::SVGAttributeOwnerProxy::SVGAttributeOwnerProxy): Move implementation to cpp file.
(WebCore::SVGAttributeOwnerProxy::element const): Ditto.
* svg/properties/SVGAttributeOwnerProxyImpl.h: Update for WeakPtr use.

LayoutTests:

* svg/animations/view-dependency-crash-expected.txt: Added.
* svg/animations/view-dependency-crash.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@239070 268f45cc-cd09-0410-ab3c-d52691b4dbfc