gpu: Add async pixel transfer interface, stub and tests.

gpu/command_buffer/service/common_decoder.cc

Index: gpu/command_buffer/service/common_decoder.cc
diff --git a/gpu/command_buffer/service/common_decoder.cc b/gpu/command_buffer/service/common_decoder.cc
index 74adcea0143a4e4d9a3a607aa8c76b0330966870..5a73cbd66b81ef78624724b7f75ee710f4a7e0ce 100644
--- a/gpu/command_buffer/service/common_decoder.cc
+++ b/gpu/command_buffer/service/common_decoder.cc
@@ -73,6 +73,10 @@ void* CommonDecoder::GetAddressAndCheckSize(unsigned int shm_id,
   return static_cast<int8*>(buffer.ptr) + offset;
 }
 
+Buffer CommonDecoder::GetSharedMemoryBuffer(unsigned int shm_id) {
+  return engine_->GetSharedMemoryBuffer(shm_id);
+}
+
 bool CommonDecoder::PushAddress(uint32 offset) {
   if (call_stack_.size() < kMaxStackDepth) {
     CommandAddress return_address(engine_->GetGetOffset());