Fix build error with XCode6/OSX 10.9 SDK

OpenGL/gl.h is required to be included to make the build pass.

BUG=
TEST=

Review URL: https://codereview.chromium.org/522533002

Cr-Commit-Position: refs/heads/master@{#292810}

content/browser/renderer_host/compositing_iosurface_mac.mm

@@ -7,6 +7,7 @@
 #include <OpenGL/CGLIOSurface.h>
 #include <OpenGL/CGLRenderers.h>
 #include <OpenGL/OpenGL.h>
+#include <OpenGL/gl.h>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"