Fix content build with OS X 10.9 SDK

Missing GL_TEXTURE_RECTANGLE_ARB definition if gl.h or glext.h is
not included.

Review URL: https://codereview.chromium.org/511793002

Cr-Commit-Position: refs/heads/master@{#292159}

content/common/gpu/media/vt_video_decode_accelerator.cc

@@ -4,6 +4,7 @@
 
 #include <CoreVideo/CoreVideo.h>
 #include <OpenGL/CGLIOSurface.h>
+#include <OpenGL/gl.h>
 
 #include "base/bind.h"
 #include "base/command_line.h"