Use MGLGetProcAddress to access OpenGL functions

Rather than calling GL functions directly, use MGLGetProcAddress to
retrieve the functions dynamically at runtime.

BUG= https://github.com/domokit/mojo/issues/396
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/1416133002 .
diff --git a/examples/spinning_cube/spinning_cube.h b/examples/spinning_cube/spinning_cube.h
index 0352970..e493c0b 100644
--- a/examples/spinning_cube/spinning_cube.h
+++ b/examples/spinning_cube/spinning_cube.h
@@ -11,6 +11,8 @@
 
 namespace examples {
 
+class GLInterface;
+
 class SpinningCube {
  public:
   SpinningCube();
@@ -42,6 +44,7 @@
   bool initialized_;
   uint32_t width_;
   uint32_t height_;
+  scoped_ptr<GLInterface> gl_;
   scoped_ptr<GLState> state_;
   float fling_multiplier_;
   int direction_;