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

content/browser/presentation/presentation_service_impl.h

Index: content/browser/presentation/presentation_service_impl.h
diff --git a/content/browser/presentation/presentation_service_impl.h b/content/browser/presentation/presentation_service_impl.h
index 5b4972d62db9046766f396224397e1662c7c8308..c640b6a277ee5aeee29e38465e9458ba2c2582b2 100644
--- a/content/browser/presentation/presentation_service_impl.h
+++ b/content/browser/presentation/presentation_service_impl.h
@@ -64,6 +64,7 @@ class CONTENT_EXPORT PresentationServiceImpl
   using SessionStateCallback =
       mojo::Callback<void(presentation::PresentationSessionInfoPtr,
           presentation::PresentationSessionState)>;
+  using SendMessageMojoCallback = mojo::Callback<void()>;
 
   // A helper data class used by PresentationServiceImpl to do bookkeeping
   // of currently registered screen availability listeners.
@@ -166,6 +167,11 @@ class CONTENT_EXPORT PresentationServiceImpl
       const mojo::String& presentation_url,
       const mojo::String& presentation_id,
       const NewSessionMojoCallback& callback) override;
+  void SendStringMessage(
+      const mojo::String& presentation_url,
+      const mojo::String& presentation_id,
+      const mojo::String& message,
+      const SendMessageMojoCallback& callback) override;
   void CloseSession(
       const mojo::String& presentation_url,
       const mojo::String& presentation_id) override;