Introduce RenderProcessHostObserver, use it in its first client.

content/public/test/mock_render_process_host.cc

Index: content/public/test/mock_render_process_host.cc
diff --git a/content/public/test/mock_render_process_host.cc b/content/public/test/mock_render_process_host.cc
index 20f44cca3690ebfc4bf880aa373d32fdea50029e..c1fde060e8737a09ae14718d9102b3f0cfbec3a2 100644
--- a/content/public/test/mock_render_process_host.cc
+++ b/content/public/test/mock_render_process_host.cc
@@ -67,6 +67,15 @@ void MockRenderProcessHost::RemoveRoute(int32 routing_id) {
   Cleanup();
 }
 
+void MockRenderProcessHost::AddObserver(RenderProcessHostObserver* observer) {
+  NOTREACHED();
+}
+
+void MockRenderProcessHost::RemoveObserver(
+    RenderProcessHostObserver* observer) {
+  NOTREACHED();
+}
+
 bool MockRenderProcessHost::WaitForBackingStoreMsg(
     int render_widget_id,
     const base::TimeDelta& max_delay,