With the viz process to be split out, ServerSharedBitmapManager will no longer be in the same process as content.

Currently it is used directly in non-testing code:
  -  BrowserMainLoop::PostMainMessageLoopStart() for tracing
  -  GpuProcessTransportFactory::EstablishedGpuChannel
  -  RenderProcessHostImpl to access viz::SharedBitmapAllocationNotifierImpl
  -  RenderWidgetHostImpl::OnShowDisambiguationPopup to grab a SharedBitmap

These need to be updated to work with just a SharedBitmapManager object. Which will be backed by the process' ClientSharedBitmapManager, but that will be hidden away.

While we are at is, we should remove the singleton nature of ServerSharedBitmapManager