[PresentationAPI] Implementing send() from WebPresentationClient to the PresentationService.

content/renderer/presentation/presentation_dispatcher.cc

Index: content/renderer/presentation/presentation_dispatcher.cc
diff --git a/content/renderer/presentation/presentation_dispatcher.cc b/content/renderer/presentation/presentation_dispatcher.cc
index eec933b874dba7d3ebdf510d6dc3833d3e3a74e5..f2a6bc72c28a47c5b8ce86f30f1dc8c76d39e733 100644
--- a/content/renderer/presentation/presentation_dispatcher.cc
+++ b/content/renderer/presentation/presentation_dispatcher.cc
@@ -120,6 +120,17 @@ void PresentationDispatcher::joinSession(
           base::Owned(callback)));
 }
 
+void PresentationDispatcher::postMessage(
+    const blink::WebString& presentationUrl,
+    const blink::WebString& presentationId,
+    const blink::WebString& message) {
+  ConnectToPresentationServiceIfNeeded();
+  presentation_service_->PostMessage(
+      presentationUrl.utf8(),
+      presentationId.utf8(),
+      message.utf8());
+}
+
 void PresentationDispatcher::closeSession(
     const blink::WebString& presentationUrl,
     const blink::WebString& presentationId) {