Make GL calls go through subclassable class.

ui/gl/gl_implementation_linux.cc

Index: ui/gl/gl_implementation_linux.cc
diff --git a/ui/gl/gl_implementation_linux.cc b/ui/gl/gl_implementation_linux.cc
index e5ca5f698dc21bcdd60a73bc8519afaffb84a58f..dc81fbc8598a68a4924532e40784517dbec8290d 100644
--- a/ui/gl/gl_implementation_linux.cc
+++ b/ui/gl/gl_implementation_linux.cc
@@ -12,7 +12,11 @@
 #include "base/path_service.h"
 #include "base/threading/thread_restrictions.h"
 #include "ui/gl/gl_bindings.h"
+#include "ui/gl/gl_egl_api_implementation.h"
+#include "ui/gl/gl_gl_api_implementation.h"
+#include "ui/gl/gl_glx_api_implementation.h"
 #include "ui/gl/gl_implementation.h"
+#include "ui/gl/gl_osmesa_api_implementation.h"
 #include "ui/gl/gl_switches.h"
 
 namespace gfx {
@@ -165,8 +169,8 @@ bool InitializeGLBindings(GLImplementation implementation) {
 
       // These two functions take single precision float rather than double
       // precision float parameters in GLES.
-      ::gfx::g_glClearDepth = MarshalClearDepthToClearDepthf;
-      ::gfx::g_glDepthRange = MarshalDepthRangeToDepthRangef;
+      ::gfx::g_driver_gl.fn.glClearDepthFn = MarshalClearDepthToClearDepthf;
+      ::gfx::g_driver_gl.fn.glDepthRangeFn = MarshalDepthRangeToDepthRangef;
       break;
     }
     case kGLImplementationMockGL: {