[Application] Add the check for the return value.

This patch is to add the check for the return value of FromID() and
GetStoragePartition().

Related to XWALK-2928.
yes, it should be checked but I mean we could be

content::RenderProcessHost* rph =
content::RenderProcessHost::FromID(application->GetRenderProcessHostID());
if (!rph) {
// handle error.
return;
}

Further, I think it's invalid state if we cannot get 'rph' from application, so I think it should be just 
CHECK(rph);