Make stream captures work on canvases that are not in the DOM.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 7b818abb555a4131e83cd66c173a4d60c96d80b2..ae598c0253cc9674c4c93b2accfc9f45b6d54144 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -1105,10 +1105,7 @@ void RenderWidgetCompositor::RequestScheduleAnimation() {
 
 void RenderWidgetCompositor::DidSubmitCompositorFrame() {}
 
-void RenderWidgetCompositor::DidLoseCompositorFrameSink() {
-  // The CompositorFrameSink is not lost in layout tests (single thread mode).
-  NOTREACHED();
-}
+void RenderWidgetCompositor::DidLoseCompositorFrameSink() {}
 
 void RenderWidgetCompositor::SetFrameSinkId(
     const cc::FrameSinkId& frame_sink_id) {