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 3bc288d69076af5e7f72ae722f9eda75cf369b4b..8abb77d6dfaea2cb1a41d27741220a635e374f30 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -1115,10 +1115,7 @@ 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) {