Switch RenderFrameHostManager to use RenderProcessHostObserver, remove NOTIFICATION_RENDERER_PROCES…

content/public/browser/render_process_host_observer.h

Index: content/public/browser/render_process_host_observer.h
diff --git a/content/public/browser/render_process_host_observer.h b/content/public/browser/render_process_host_observer.h
index 9414f57a6e6d3e42d9996be33fd1f89fc89d2a75..c4d4811ca91e16f8e2a86ce75ce2e48ed58a80fd 100644
--- a/content/public/browser/render_process_host_observer.h
+++ b/content/public/browser/render_process_host_observer.h
@@ -17,6 +17,10 @@ class RenderProcessHost;
 // in RenderProcessHost lifecycle events.
 class CONTENT_EXPORT RenderProcessHostObserver {
  public:
+  // This method is invoked when the process is going to exit and should not be
+  // used for further navigations.

[ncarter (slow), 2015/07/21 19:07:15]

I think this gets a little confusing, especially with how RenderProcessExited
has STILL_RUNNING as an option for |status|. So, the more comments here the
better -- in particular, assume that the reader needs to tell the difference
between all the functions here. My mental model is something like:

 - WillExit: don't place any new objects in this renderer process (unless ...
it's recreated???)
 - Exited: you will never again hear from objects that were in this renderer
process (... but watch out, they might be reborn under the same ids in a new
child process!)
 - HostDestroyed: destroy any objects tied to the rph, that aren't tied to the
lifetime of any single child process.

[Avi (use Gerrit), 2015/07/21 20:45:16]

Done.

+  virtual void RenderProcessWillExit(RenderProcessHost* host) {}
+
   // This method is invoked when the process of the observed RenderProcessHost
   // exits (either normally or with a crash). To determine if the process closed
   // normally or crashed, examine the |status| parameter.