Bug 1875528 - Part 3: Remove unnecessary VsyncChild strong reference in BrowserChild, r=smaug

This reference created a cycle after the strong Manager() reference added in an earlier part.

With the reference removed, the VsyncChild will be kept alive via the IPC link
instead, and will be torn down when the IPDL actor is destroyed.

Differential Revision: https://phabricator.services.mozilla.com/D198626

dom/ipc/BrowserChild.cpp

@@ -2150,18 +2150,23 @@ bool BrowserChild::DeallocPDocAccessibleChild(
 #endif
 
 RefPtr<VsyncMainChild> BrowserChild::GetVsyncChild() {
-  // Initializing mVsyncChild here turns on per-BrowserChild Vsync for a
+  // Initializing VsyncMainChild here turns on per-BrowserChild Vsync for a
   // given platform. Note: this only makes sense if nsWindow returns a
   // window-specific VsyncSource.
 #if defined(MOZ_WAYLAND)
-  if (IsWaylandEnabled() && !mVsyncChild) {
-    mVsyncChild = MakeRefPtr<VsyncMainChild>();
-    if (!SendPVsyncConstructor(mVsyncChild)) {
-      mVsyncChild = nullptr;
+  if (IsWaylandEnabled()) {
+    if (auto* actor = static_cast<VsyncMainChild*>(
+            LoneManagedOrNullAsserts(ManagedPVsyncChild()))) {
+      return actor;
     }
+    auto actor = MakeRefPtr<VsyncMainChild>();
+    if (!SendPVsyncConstructor(actor)) {
+      return nullptr;
+    }
+    return actor;
   }
 #endif
-  return mVsyncChild;
+  return nullptr;
 }
 
 mozilla::ipc::IPCResult BrowserChild::RecvLoadRemoteScript(

dom/ipc/BrowserChild.h

@@ -748,8 +748,6 @@ class BrowserChild final : public nsMessageManagerScriptExecutor,
   Maybe<bool> mLayersConnectRequested;
   EffectsInfo mEffectsInfo;
 
-  RefPtr<VsyncMainChild> mVsyncChild;
-
   RefPtr<APZEventState> mAPZEventState;
 
   // Position of client area relative to the outer window