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 df35d608f3048add0b33404d33efbbace8f39a5e..bdc7cf0f64b2f160f352a60eca1eb6af79c590b5 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -1111,10 +1111,7 @@ 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) {