[PresentationAPI] Added plumbing for start/joinSession from JS to platform

Source/modules/presentation/PresentationSession.cpp

Index: Source/modules/presentation/PresentationSession.cpp
diff --git a/Source/modules/presentation/PresentationSession.cpp b/Source/modules/presentation/PresentationSession.cpp
index a7920a0747f08f762c06e42e10cc656b282f16e3..97012e683c7d10462acdf70f8d5fd755cb5c1cf7 100644
--- a/Source/modules/presentation/PresentationSession.cpp
+++ b/Source/modules/presentation/PresentationSession.cpp
@@ -5,12 +5,17 @@
 #include "config.h"
 #include "modules/presentation/PresentationSession.h"
 
+#include "core/dom/Document.h"
+#include "core/frame/LocalFrame.h"
 #include "modules/EventTargetModules.h"
+#include "public/platform/modules/presentation/WebPresentationSessionClient.h"
 
 namespace blink {
 
-PresentationSession::PresentationSession(ExecutionContext* executionContext)
-    : ContextLifecycleObserver(executionContext)
+PresentationSession::PresentationSession(LocalFrame* frame, WebPresentationSessionClient* client)
+    : DOMWindowProperty(frame)
+    , m_id(client->getId())

[mlamouri (slow - plz ping), 2015/02/25 20:51:06]

So if you only need the id, maybe you could only pass the ID here and in the
callback. WebCallbacks usually own the pointers they are given. By passing
|client|, it is expected that the PresentationSession will own it and it's a bit
confusing.

[whywhat, 2015/02/25 21:04:14]

The client needs to outlive the callback since it is used to communicate with
the session object on the browser process side via Mojo.

[mlamouri (slow - plz ping), 2015/02/25 21:08:54]

Understood. That's why you should pass the id only, not the whole client. Unless
you will need the client later? (which would be problematic)

[whywhat, 2015/02/25 22:46:52]

Ok, I'll pass the presentation id and will use it to look the
WebPresentationSessionClient up later on.

+    , m_state("disconnected")
 {
 }
 
@@ -19,9 +24,9 @@ PresentationSession::~PresentationSession()
 }
 
 // static
-PresentationSession* PresentationSession::create(ExecutionContext* executionContext)
+PresentationSession* PresentationSession::create(LocalFrame* frame, WebPresentationSessionClient* client)
 {
-    return new PresentationSession(executionContext);
+    return new PresentationSession(frame, client);
 }
 
 const AtomicString& PresentationSession::interfaceName() const
@@ -31,13 +36,14 @@ const AtomicString& PresentationSession::interfaceName() const
 
 ExecutionContext* PresentationSession::executionContext() const
 {
-    return ContextLifecycleObserver::executionContext();
-}
+    if (!frame())
+        return nullptr;
+    return frame()->document();}
 
 DEFINE_TRACE(PresentationSession)
 {
     RefCountedGarbageCollectedEventTargetWithInlineData<PresentationSession>::trace(visitor);
-    ContextLifecycleObserver::trace(visitor);
+    DOMWindowProperty::trace(visitor);
 }
 
 void PresentationSession::postMessage(const String& message)