Enable asynchronous glReadPixels on Windows.

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 33bd94b04561b74640f81579987f741c45a9b254..371395625e41caacc4c22528ef880de6e185287c 100644
--- a/gpu/command_buffer/client/buffer_tracker.h
+++ b/gpu/command_buffer/client/buffer_tracker.h
@@ -28,12 +28,14 @@ class GLES2_IMPL_EXPORT BufferTracker {
            unsigned int size,
            int32 shm_id,
            uint32 shm_offset,
-           void* address)
+           void* address,
+           unsigned int metadata_size)
         : id_(id),
           size_(size),
           shm_id_(shm_id),
           shm_offset_(shm_offset),
           address_(address),
+          metadata_size_(metadata_size),
           mapped_(false),
           last_usage_token_(0),
           last_async_upload_token_(0) {
@@ -59,6 +61,22 @@ class GLES2_IMPL_EXPORT BufferTracker {
       return address_;
     }
 
+    unsigned int metadata_size() const {
+      return metadata_size_;
+    }
+
+    int32 metadata_shm_id() const {
+      return shm_id_;
+    }
+
+    uint32 metadata_shm_offset() const {
+      return shm_offset_ + size_;
+    }
+
+    void* metadata_address() const {
+      return static_cast<uint8*>(address_) + size_;
+    }
+
     void set_mapped(bool mapped) {
       mapped_ = mapped;
     }
@@ -92,6 +110,7 @@ class GLES2_IMPL_EXPORT BufferTracker {
     int32 shm_id_;
     uint32 shm_offset_;
     void* address_;
+    unsigned int metadata_size_;
     bool mapped_;
     int32 last_usage_token_;
     GLuint last_async_upload_token_;
@@ -100,7 +119,7 @@ class GLES2_IMPL_EXPORT BufferTracker {
   BufferTracker(MappedMemoryManager* manager);
   ~BufferTracker();
 
-  Buffer* CreateBuffer(GLuint id, GLsizeiptr size);
+  Buffer* CreateBuffer(GLuint id, GLsizeiptr size, GLsizeiptr metadata_size);
   Buffer* GetBuffer(GLuint id);
   void RemoveBuffer(GLuint id);