Implement client side PBOs for glReadPixel

gpu/command_buffer/client/buffer_tracker.h

Index: gpu/command_buffer/client/buffer_tracker.h
diff --git a/gpu/command_buffer/client/buffer_tracker.h b/gpu/command_buffer/client/buffer_tracker.h
index 54f5bc793d3a785929f12a93e19cfe430ab866a8..f8fc057278c79d222d5b7183dd06147f4810a30d 100644
--- a/gpu/command_buffer/client/buffer_tracker.h
+++ b/gpu/command_buffer/client/buffer_tracker.h
@@ -34,7 +34,8 @@ class GLES2_IMPL_EXPORT BufferTracker {
           shm_id_(shm_id),
           shm_offset_(shm_offset),
           address_(address),
-          mapped_(false) {
+          mapped_(false),
+          transfer_ready_token_(0) {
     }
 
     GLenum id() const {
@@ -65,6 +66,14 @@ class GLES2_IMPL_EXPORT BufferTracker {
       return mapped_;
     }
 
+    void set_transfer_ready_token(int token) {
+      transfer_ready_token_ = token;
+    }
+
+    uint32 transfer_ready_token() const {
+      return transfer_ready_token_;
+    }
+
    private:
     friend class BufferTracker;
     friend class BufferTrackerTest;
@@ -75,6 +84,7 @@ class GLES2_IMPL_EXPORT BufferTracker {
     uint32 shm_offset_;
     void* address_;
     bool mapped_;
+    int32 transfer_ready_token_;
   };
 
   BufferTracker(MappedMemoryManager* manager);