Use PBuffer to make TextureImageTransportSurface current.

content/common/gpu/texture_image_transport_surface.h

Index: content/common/gpu/texture_image_transport_surface.h
diff --git a/content/common/gpu/texture_image_transport_surface.h b/content/common/gpu/texture_image_transport_surface.h
index 6764fc31651694f487aa5f0ef3fde98b13b7ae05..19ed88c32fb7615465c99c72611cf0ac3d800e15 100644
--- a/content/common/gpu/texture_image_transport_surface.h
+++ b/content/common/gpu/texture_image_transport_surface.h
@@ -110,6 +110,10 @@ class TextureImageTransportSurface :
   gfx::GLSurfaceHandle handle_;
   GpuCommandBufferStub* parent_stub_;
 
+  // The offscreeb surface used to make the context current. However note that
+  // the actual rendering is always redirected to an FBO.
+  scoped_refptr<GLSurface> surface_;
+
   DISALLOW_COPY_AND_ASSIGN(TextureImageTransportSurface);
 };