Re-land: cc: Implement shared worker contexts.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index e17431d89c00f4b35cec785252110860e273ec28..444ed00f9e93406d70e636b6de04badab9651e64 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -434,6 +434,8 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   int meta_information_sequence_number_;
   scoped_ptr<RenderingStatsInstrumentation> rendering_stats_instrumentation_;
 
+  scoped_ptr<OutputSurface> new_output_surface_;

[danakj, 2015/09/11 19:49:55]

What's the reasoning for these variables? Why own the output surface here? These
own the output surface for the entire time it's used by the compositor? Can you
explain in some comments?

[reveman, 2015/09/11 22:26:13]

This is just moving ownership from the compositor thread to the main thread to
ensure that the output surface is always destroyed on the correct thread. I made
sure current_output_surface_ is reset if init fails and added a comment here in
latest patch. Also updated the description to include the most important part of
this patch: output surfaces now have to be destroyed on the thread they were
created on.

+  scoped_ptr<OutputSurface> current_output_surface_;
   bool output_surface_lost_;
 
   scoped_refptr<Layer> root_layer_;