Bug 1881738 - Don't send invalid remote texture owner id when WebGLChild is destroyed. r=aosmond

It seems like Canvas2D and WebGPU already avoid sending over invalid owner ids in GetFrontBuffer.
WebGL, however, can still blindly send an invalid texture owner id when the actor is being destroyed,
but the WebGL context is still alive.

Differential Revision: https://phabricator.services.mozilla.com/D206276

dom/canvas/ClientWebGLContext.cpp

@@ -567,6 +567,11 @@ Maybe<layers::SurfaceDescriptor> ClientWebGLContext::GetFrontBuffer(
     info.flushesSinceLastCongestionCheck = 0;
     info.congestionCheckGeneration++;
 
+    // If the actor is destroyed, don't send over invalid texture owner id.
+    if (!child->CanSend()) {
+      return {};
+    }
+
     return Some(layers::SurfaceDescriptorRemoteTexture(*mLastRemoteTextureId,
                                                        *mRemoteTextureOwnerId));
   }