Implemented window class on linux. Standalone demos are working on linux now.

gpu/demos/framework/main_exe.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/at_exit.h"
 #include "base/logging.h"
 #include "gpu/demos/framework/window.h"
 
+#if defined(OS_LINUX)
+#include <gtk/gtk.h>
+#endif  // OS_LINUX
+
 namespace {
 static const int kWindowWidth = 512;
 static const int kWindowHeight = 512;
 }  // namespace.
 
 int main(int argc, char *argv[]) {
+#if defined(OS_LINUX)
+  gtk_init(&argc, &argv);
+#endif  // OS_LINUX
+
   // AtExitManager is used by singleton classes to delete themselves when
   // the program terminates.
   base::AtExitManager at_exit_manager_;
 
   gpu::demos::Window window;
   CHECK(window.Init(kWindowWidth, kWindowHeight));
 
   window.MainLoop();
   return EXIT_SUCCESS;
 }
+