LocalResourceHeap is the only super class of ResourceHeap so it is cleaner to move all the implementation of LocalResourceHeap to ResourceHeap.

All the calls to the add() and get() functions of QualifiedResourceHeap come from RemoteRenderingBackend and RemoteDisplayListRecorder via RemoteResourceCache. These two callers qualify the RenderingResourceIdentifier with m_gpuConnectionToWebProcess->webProcessIdentifier() of the RemoteRenderingBackend.

But RemoteRenderingBackend never changes its m_gpuConnectionToWebProcess since it is defined to be `Ref<GPUConnectionToWebProcess>`. And GPUConnectionToWebProcess never changes its m_webProcessIdentifier since it is defined to be `const WebCore::ProcessIdentifier`. So the resources stored by QualifiedResourceHeap should have the same ProcessIdentifier.

This will make replaying a whole DisplayList in GPU Process be straightforward once the DisplayList items and the resources are transferred from Web Content Process to the GPU Process. The DisplayList::Replayer can be initialized with the decoded DisplayList items and the ResourceHeap of the RemoteResourceCache.