[PresentationAPI] Plumbing send() from PresentationSession IDL to platform/.

public/platform/modules/presentation/WebPresentationClient.h

Index: public/platform/modules/presentation/WebPresentationClient.h
diff --git a/public/platform/modules/presentation/WebPresentationClient.h b/public/platform/modules/presentation/WebPresentationClient.h
index ab82031d5e09e68366df0976027d33d106e6874b..c5adcd4e500ce03da5e18545b639437362d1f008 100644
--- a/public/platform/modules/presentation/WebPresentationClient.h
+++ b/public/platform/modules/presentation/WebPresentationClient.h
@@ -40,6 +40,18 @@ public:
     // The ownership of the |callbacks| argument is transferred to the embedder.
     virtual void joinSession(const WebString& presentationUrl, const WebString& presentationId, WebPresentationSessionClientCallbacks*) = 0;
 
+    // Called when the frame requests to send String message to an existing session.
+    virtual void postMessage(const WebString& url, const WebString& presentationId, const WebString& message)
+    {
+        // TODO(s.singapati): make this pure virtual once Chromium has the implementation.
+        BLINK_ASSERT_NOT_REACHED();
+    }
+    virtual void postMessage(const WebString& url, const WebString& presentationId, const char* data, size_t length)
+    {
+        // TODO(s.singapati): make this pure virtual once Chromium has the implementation.
+        BLINK_ASSERT_NOT_REACHED();
+    }
+
     // Called when the frame requests to close an existing session.
     virtual void closeSession(const WebString& url, const WebString& presentationId) = 0;
 };