Bug 1638123 - Call WebGLContext::ClearVRFrame when ending WebXR session. r=jgilbert,kip,daoshengmu

In WebVR we call WebGLContext::ClearVRFrame when the presentation ends. We didn't have the equivalent call in WebXR. It helps to reduce memory usage earlier when exiting WebXR and reduces potential deadlocks if the ExternalVR client didn't call SurfaceTexture->ReleaseTexImage() correctly.

Differential Revision: https://phabricator.services.mozilla.com/D75417

dom/canvas/XRWebGLLayer.cpp

@@ -259,7 +259,10 @@ HTMLCanvasElement* XRWebGLLayer::GetCanvas() {
   return mWebGL->GetParentObject();
 }
 
-void XRWebGLLayer::SessionEnded() { DeleteFramebuffer(); }
+void XRWebGLLayer::SessionEnded() {
+  DeleteFramebuffer();
+  mWebGL->ClearVRFrame();
+}
 
 }  // namespace dom
 }  // namespace mozilla