Bug 1498092 - Add necessary forced frame rendering r=nical

gfx/layers/ipc/CompositorBridgeParent.cpp

@@ -645,7 +645,7 @@ mozilla::ipc::IPCResult
 CompositorBridgeParent::RecvForcePresent()
 {
   if (mWrBridge) {
-    mWrBridge->ScheduleGenerateFrame();
+    mWrBridge->ScheduleForcedGenerateFrame();
   }
   // During the shutdown sequence mLayerManager may be null
   if (mLayerManager) {

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -1390,17 +1390,22 @@ WebRenderBridgeParent::UpdateWebRender(CompositorVsyncScheduler* aScheduler,
 mozilla::ipc::IPCResult
 WebRenderBridgeParent::RecvScheduleComposite()
 {
-  if (mDestroyed) {
+  ScheduleGenerateFrame();
   return IPC_OK();
+}
+
+void
+WebRenderBridgeParent::ScheduleForcedGenerateFrame()
+{
+  if (mDestroyed) {
+    return;
   }
 
-  // Force frame rendering during next frame generation.
   wr::TransactionBuilder fastTxn(/* aUseSceneBuilderThread */ false);
   fastTxn.InvalidateRenderedFrame();
   mApi->SendTransaction(fastTxn);
 
   ScheduleGenerateFrame();
-  return IPC_OK();
 }
 
 mozilla::ipc::IPCResult

gfx/layers/wr/WebRenderBridgeParent.h

@@ -186,6 +186,14 @@ public:
    */
   void ScheduleGenerateFrame();
 
+  /**
+   * Schedule forced frame rendering at next composite timing.
+   *
+   * WebRender could skip frame rendering if there is no update.
+   * This function is used to force rendering even when there is not update.
+   */
+  void ScheduleForcedGenerateFrame();
+
   wr::Epoch UpdateWebRender(CompositorVsyncScheduler* aScheduler,
                             wr::WebRenderAPI* aApi,
                             AsyncImagePipelineManager* aImageMgr,