Bug 1808155 Part 3: Make NativeLayerCA report when RenderCompositorNative is creating new video surfaces. r=mstange

This additional logging message will help to eliminate the false positives
of AttachExternalImage or SetRootWindowIsFullscreen being blamed for
causing the video layer to be rebuilt. If the layer itself is new, a video
layer rebuild is unavoidable.

Differential Revision: https://phabricator.services.mozilla.com/D175399

gfx/layers/NativeLayerCA.mm

@@ -755,7 +755,15 @@ NativeLayerCA::NativeLayerCA(const IntSize& aSize, bool aIsOpaque,
 }
 
 NativeLayerCA::NativeLayerCA(bool aIsOpaque)
-    : mMutex("NativeLayerCA"), mSurfacePoolHandle(nullptr), mIsOpaque(aIsOpaque) {}
+    : mMutex("NativeLayerCA"), mSurfacePoolHandle(nullptr), mIsOpaque(aIsOpaque) {
+#ifdef NIGHTLY_BUILD
+  if (StaticPrefs::gfx_core_animation_specialize_video_log()) {
+    NSLog(@"VIDEO_LOG: NativeLayerCA: %p is being created to host video, which will force a video "
+          @"layer rebuild.",
+          this);
+  }
+#endif
+}
 
 CGColorRef CGColorCreateForDeviceColor(gfx::DeviceColor aColor) {
   if (StaticPrefs::gfx_color_management_native_srgb()) {