Implement TransformFeedbackManager in GPU command buffer.

gpu/command_buffer/service/context_state.h

Index: gpu/command_buffer/service/context_state.h
diff --git a/gpu/command_buffer/service/context_state.h b/gpu/command_buffer/service/context_state.h
index 80687b7db51fdf7fccc389c3b18b3745788e63b3..3a44730c40b37e49128c4a1692aac8dc3052fc90 100644
--- a/gpu/command_buffer/service/context_state.h
+++ b/gpu/command_buffer/service/context_state.h
@@ -29,6 +29,7 @@ class Framebuffer;
 class Logger;
 class Program;
 class Renderbuffer;
+class TransformFeedback;
 
 // State associated with each texture unit.
 struct GPU_EXPORT TextureUnit {
@@ -188,6 +189,7 @@ struct GPU_EXPORT ContextState {
   void RestoreGlobalState(const ContextState* prev_state) const;
   void RestoreProgramBindings() const;
   void RestoreRenderbufferBindings();
+  void RestoreTransformFeedbackBindings(const ContextState* prev_state);
   void RestoreTextureUnitBindings(
       GLuint unit, const ContextState* prev_state) const;
 
@@ -273,6 +275,14 @@ struct GPU_EXPORT ContextState {
   // Which samplers are bound to each texture unit;
   std::vector<scoped_refptr<Sampler>> sampler_units;
 
+  // We create a transform feedback as the default one per ES3 enabled context
+  // instead of using GL's default one to make context switching easier.
+  // For other context, we will never change the default transform feedback's
+  // states, so we can just use the GL's default one.
+  scoped_refptr<TransformFeedback> default_transform_feedback;
+
+  scoped_refptr<TransformFeedback> bound_transform_feedback;
+
   // The values for each attrib.
   std::vector<Vec4> attrib_values;