Ensure GL initialization only happens once, and provide common init path

ui/gl/gl_implementation.h

Index: ui/gl/gl_implementation.h
diff --git a/ui/gl/gl_implementation.h b/ui/gl/gl_implementation.h
index 045101c1d54dc61e342ef60089edc8f5b74e04c9..7dc736a4dbe8c6bad926c22e1fc6bdb5c61c3fdf 100644
--- a/ui/gl/gl_implementation.h
+++ b/ui/gl/gl_implementation.h
@@ -42,12 +42,12 @@ typedef void* (*GLGetProcAddressProc)(const char* name);
 #endif
 
 // Initialize a particular GL implementation.
-GL_EXPORT bool InitializeStaticGLBindings(GLImplementation implementation);
+bool InitializeStaticGLBindings(GLImplementation implementation);
 
 // Initialize function bindings that depend on the context for a GL
 // implementation.
-GL_EXPORT bool InitializeDynamicGLBindings(GLImplementation implementation,
-    GLContext* context);
+bool InitializeDynamicGLBindings(GLImplementation implementation,
+                                 GLContext* context);
 
 // Initialize Debug logging wrappers for GL bindings.
 void InitializeDebugGLBindings();
@@ -55,7 +55,7 @@ void InitializeDebugGLBindings();
 // Initialize stub methods for drawing operations in the GL bindings.
 void InitializeNullDrawGLBindings();
 
-GL_EXPORT void ClearGLBindings();
+void ClearGLBindings();
 
 // Set the current GL implementation.
 GL_EXPORT void SetGLImplementation(GLImplementation implementation);