[Extensions] Handle ExtensionProcess crashes gracefully

This PR enables us to gracefully handle an EP crash within the BrowserProcess.

The first commit adds a crash extension and a EP crash "demo". One can try it and see that even after crashing EP you still have RP and BP in place.

The second commit makes us use zygote to Launch EP from now on, on Linux. This gives us guarantee that OnProcessCrashed will always get called so we can act upon an EP crash. For doing that, I've added the XWalkExtensionProcessHost::Observer interface, so XWalkExtensionService can follow what happens with EP. (that entity is, so far, the only observer we need)