Replace context parenting by sharing through mailboxes

ppapi/proxy/ppapi_command_buffer_proxy.cc

Index: ppapi/proxy/ppapi_command_buffer_proxy.cc
diff --git a/ppapi/proxy/ppapi_command_buffer_proxy.cc b/ppapi/proxy/ppapi_command_buffer_proxy.cc
index 04c4468d8be04ff4e2f46458cb9abeeabcc91594..8ca17e442d1e20b7b6f2d70b25c3e6caaa8269d8 100644
--- a/ppapi/proxy/ppapi_command_buffer_proxy.cc
+++ b/ppapi/proxy/ppapi_command_buffer_proxy.cc
@@ -45,11 +45,7 @@ bool PpapiCommandBufferProxy::Echo(const base::Closure& callback) {
   return false;
 }
 
-bool PpapiCommandBufferProxy::SetParent(
-    CommandBufferProxy* parent_command_buffer,
-    uint32 parent_texture_id) {
-  // TODO(fsamuel): Need a proper implementation of this to support offscreen
-  // contexts in the guest renderer (WebGL, canvas, etc).
+bool PpapiCommandBufferProxy::ProduceFrontBuffer(const gpu::Mailbox& mailbox) {
   NOTIMPLEMENTED();
   return false;
 }