gpu/linux: create a child window to control resize and avoid flashes

ui/gl/gl_surface_glx.h

Index: ui/gl/gl_surface_glx.h
diff --git a/ui/gl/gl_surface_glx.h b/ui/gl/gl_surface_glx.h
index 8b199f734ef5ada690daa8a46686bcaaefc9e940..58088a75c130759c55c555237aba425d5373470e 100644
--- a/ui/gl/gl_surface_glx.h
+++ b/ui/gl/gl_surface_glx.h
@@ -76,6 +76,9 @@ class GL_EXPORT NativeViewGLSurfaceGLX : public GLSurfaceGLX {
   // Window passed in at creation. Always valid.
   gfx::AcceleratedWidget parent_window_;
 
+  // Child window, used to control resizes so that they're in-order with GL.
+  gfx::AcceleratedWidget window_;
+
   void* config_;
   gfx::Size size_;