[Extension][Runtime] Proper Crash Handling

This PR adds two commits related to ExtensionProcess (EP) and RenderProcess(RP) crash handling. The rationale here, in summary, is that what defines the crash of a WebApp nowadays is either the crash of EP or the crash of RP. In both cases we need to shutdown the remaining pieces of the app properly.

The first commit adds the necessary infra so we can be warned when the RenderProcess dies. We use that then to warn XWalkExtensionService so it can properly shutdown the ExtensionProcess, the in process XWalkExtensionServer and the associated ExtensionData.

The second commit extends the previously added XWalkExtensionService::OnExtensionProcessDied() so it also do the proper clean up for that specific case, including the shutdown of the RenderProcess.