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

Source/core/dom/ContextLifecycleObserver.cpp

Index: Source/core/dom/ContextLifecycleObserver.cpp
diff --git a/Source/core/dom/ContextLifecycleObserver.cpp b/Source/core/dom/ContextLifecycleObserver.cpp
index 8f158361e75697254cb1e5646ecff63f64ce6c5d..2d08801b44d3ec2326b4fb848a309c91aa97cf3c 100644
--- a/Source/core/dom/ContextLifecycleObserver.cpp
+++ b/Source/core/dom/ContextLifecycleObserver.cpp
@@ -27,19 +27,18 @@
 #include "config.h"
 #include "core/dom/ContextLifecycleObserver.h"
 
-#include "core/dom/ContextLifecycleNotifier.h"
 #include "core/dom/ExecutionContext.h"
 
 namespace blink {
 
-template<> void observeContext(ExecutionContext* context, LifecycleObserver<ExecutionContext>* observer)
+template<> void observerContext(ExecutionContext* context, LifecycleObserver<ExecutionContext>* observer)
 {
-    context->addObserver(observer);
+    context->wasObservedBy(observer);
 }
 
-template<> void unobserveContext(ExecutionContext* context, LifecycleObserver<ExecutionContext>* observer)
+template<> void unobserverContext(ExecutionContext* context, LifecycleObserver<ExecutionContext>* observer)
 {
-    context->removeObserver(observer);
+    context->wasUnobservedBy(observer);
 }
 
 ContextLifecycleObserver::ContextLifecycleObserver(ExecutionContext* executionContext, Type type)