Remove windows/mac/ios specific code from //ui

ui/gl/scoped_cgl.cc

-// Copyright 2014 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/logging.h"
-#include "ui/gl/scoped_cgl.h"
-
-namespace gfx {
-
-ScopedCGLSetCurrentContext::ScopedCGLSetCurrentContext(CGLContextObj context) {
-  CGLContextObj previous_context = CGLGetCurrentContext();
-  // It is possible for the previous context to have a zero reference count,
-  // because making a context current does not increment the reference count.
-  // In that case, do not restore the previous context.
-  if (previous_context && CGLGetContextRetainCount(previous_context)) {
-    previous_context_.reset(previous_context, base::scoped_policy::RETAIN);
-  }
-  CGLError error = CGLSetCurrentContext(context);
-  DCHECK_EQ(error, kCGLNoError) << "CGLSetCurrentContext should never fail";
-}
-
-ScopedCGLSetCurrentContext::~ScopedCGLSetCurrentContext() {
-  CGLError error = CGLSetCurrentContext(previous_context_);
-  DCHECK_EQ(error, kCGLNoError) << "CGLSetCurrentContext should never fail";
-}
-
-}  // namespace gfx