Before bug 242497, we would rebuild the SVGResources for a client SVG element whenever we call into its RenderObject's layout function.  This was wasteful if we know that they layout change doesn't affect the resource, so in bug 242497 (and in the bugs I'm working under bug 242420), I am making the various RenderSVGResource handle caching data based on the layout information of the client.

When a RenderSVGResource is removed (e.g. because its render tree is being re-created, or the element is removed), we remove it from all clients' SVGResources and mark the clients as needing layout.  Now that we no longer unconditionally rebuild the SVGResources for the client under its layout function, we need another way to indicate that it needs to be rebuilt.