gl: Move platform-specific EGL code into separate files

ui/gl/gl_surface_osmesa.cc

Index: ui/gl/gl_surface_osmesa.cc
diff --git a/ui/gl/gl_surface_osmesa.cc b/ui/gl/gl_surface_osmesa.cc
index 9a556bc9982d528dcfb40aa81fc08e861e454b27..8018fe83824dcc20ed7adf6ad93f8251b7914979 100644
--- a/ui/gl/gl_surface_osmesa.cc
+++ b/ui/gl/gl_surface_osmesa.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "base/logging.h"
+#include "third_party/mesa/src/include/GL/osmesa.h"
 #include "ui/gl/gl_bindings.h"
 #include "ui/gl/gl_context.h"
 #include "ui/gl/gl_surface_osmesa.h"
@@ -82,4 +83,13 @@ GLSurfaceOSMesa::~GLSurfaceOSMesa() {
   Destroy();
 }
 
+bool GLSurfaceOSMesaHeadless::IsOffscreen() { return false; }
+
+bool GLSurfaceOSMesaHeadless::SwapBuffers() { return true; }
+
+GLSurfaceOSMesaHeadless::GLSurfaceOSMesaHeadless()
+    : GLSurfaceOSMesa(OSMESA_BGRA, gfx::Size(1, 1)) {}
+
+GLSurfaceOSMesaHeadless::~GLSurfaceOSMesaHeadless() { Destroy(); }
+
 }  // namespace gfx