Add an API for posting external tasks to compositor thread. Contribut…

…es to JB#30162.

This is a generic API similar to already available EmbedLiteApp::PostTask.
The main difference is, as the function name suggests,
EmbedLiteApp::PostCompostiorTask schedules the callback function to be
executed on gecko compositor thread. In case the compositor was not yet
created by the engine the function returns nullptr.

embedding/embedlite/EmbedLiteApp.cpp

@@ -13,6 +13,7 @@
 #include "base/message_loop.h"               // for MessageLoop
 
 #include "mozilla/embedlite/EmbedLiteAPI.h"
+#include "mozilla/layers/CompositorParent.h"
 
 #include "EmbedLiteUILoop.h"
 #include "EmbedLiteSubThread.h"
@@ -118,6 +119,27 @@ EmbedLiteApp::PostTask(EMBEDTaskCallback callback, void* userData, int timeout)
   return (void*)newTask;
 }
 
+void*
+EmbedLiteApp::PostCompositorTask(EMBEDTaskCallback callback, void* userData, int timeout)
+{
+  if (!mozilla::layers::CompositorParent::CompositorLoop()) {
+    // Can't post compositor task if gecko compositor thread has not been initialized, yet.
+    return nullptr;
+  }
+
+  CancelableTask* newTask = NewRunnableFunction(callback, userData);
+  MessageLoop* compositorLoop = mozilla::layers::CompositorParent::CompositorLoop();
+  MOZ_ASSERT(compositorLoop);
+
+  if (timeout) {
+    compositorLoop->PostDelayedTask(FROM_HERE, newTask, timeout);
+  } else {
+    compositorLoop->PostTask(FROM_HERE, newTask);
+  }
+
+  return (void*)newTask;
+}
+
 void
 EmbedLiteApp::CancelTask(void* aTask)
 {

embedding/embedlite/EmbedLiteApp.h

@@ -80,6 +80,7 @@ class EmbedLiteApp
 
   // Delayed post task helper for delayed functions call in main thread
   virtual void* PostTask(EMBEDTaskCallback callback, void* userData, int timeout = 0);
+  virtual void* PostCompositorTask(EMBEDTaskCallback callback, void* userData, int timeout = 0);
   virtual void CancelTask(void* aTask);
 
   // Setup profile path for embedding, or null if embedding supposed to be profile-less