Revert r189385 "Remove LifecycleContext" and r189391, r189530, r189456 that block it

Source/core/page/PageLifecycleObserver.cpp

Index: Source/core/page/PageLifecycleObserver.cpp
diff --git a/Source/core/page/PageLifecycleObserver.cpp b/Source/core/page/PageLifecycleObserver.cpp
index eb370deacd32f9c95486a3f05d9de494a96c5aab..4870f2ed1d7e1a7ebda2201c26d67ea139ff54c8 100644
--- a/Source/core/page/PageLifecycleObserver.cpp
+++ b/Source/core/page/PageLifecycleObserver.cpp
@@ -28,18 +28,17 @@
 #include "core/page/PageLifecycleObserver.h"
 
 #include "core/page/Page.h"
-#include "core/page/PageLifecycleNotifier.h"
 
 namespace blink {
 
-template<> void observeContext(Page* context, LifecycleObserver<Page>* observer)
+template<> void observerContext(Page* context, LifecycleObserver<Page>* observer)
 {
-    context->addObserver(observer);
+    context->wasObservedBy(observer);
 }
 
-template<> void unobserveContext(Page* context, LifecycleObserver<Page>* observer)
+template<> void unobserverContext(Page* context, LifecycleObserver<Page>* observer)
 {
-    context->removeObserver(observer);
+    context->wasUnobservedBy(observer);
 }
 
 PageLifecycleObserver::PageLifecycleObserver(Page* page)