mojo: Convert gles2 to the new thunking system

mojo/public/platform/native/system_thunks.cc

Index: mojo/public/platform/native/system_thunks.cc
diff --git a/mojo/public/platform/native/system_thunks.cc b/mojo/public/platform/native/system_thunks.cc
index 6fdc4f4530ff0af9c6205c6e02d4e92c0add2061..fed6db9d9b981e1bc9450074ce2542104c033438 100644
--- a/mojo/public/platform/native/system_thunks.cc
+++ b/mojo/public/platform/native/system_thunks.cc
@@ -6,6 +6,8 @@
 
 #include <assert.h>
 
+#include "mojo/public/platform/native/thunk_export.h"
+
 extern "C" {
 
 static MojoSystemThunks g_thunks = {0};
@@ -152,14 +154,6 @@ MojoResult MojoUnmapBuffer(void* buffer) {
   return g_thunks.UnmapBuffer(buffer);
 }
 
-// Call this function by looking
-// Always export this api.
-#if defined(WIN32)
-#define THUNK_EXPORT __declspec(dllexport)
-#else
-#define THUNK_EXPORT __attribute__((visibility("default")))
-#endif
-
 extern "C" THUNK_EXPORT size_t MojoSetSystemThunks(
     const MojoSystemThunks* system_thunks) {
   if (system_thunks->size >= sizeof(g_thunks))