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

ui/gl/gl_implementation_mac.cc

Index: ui/gl/gl_implementation_mac.cc
diff --git a/ui/gl/gl_implementation_mac.cc b/ui/gl/gl_implementation_mac.cc
index e7689707f209c84fea2edc0e77f8a91edfc042ac..291365d119f27e310729fbc500625369ed149fb9 100644
--- a/ui/gl/gl_implementation_mac.cc
+++ b/ui/gl/gl_implementation_mac.cc
@@ -31,8 +31,7 @@ bool InitializeStaticGLBindings(GLImplementation implementation) {
   // Prevent reinitialization with a different implementation. Once the gpu
   // unit tests have initialized with kGLImplementationMock, we don't want to
   // later switch to another GL implementation.
-  if (GetGLImplementation() != kGLImplementationNone)
-    return true;
+  DCHECK_EQ(kGLImplementationNone, GetGLImplementation());
 
   // Allow the main thread or another to initialize these bindings
   // after instituting restrictions on I/O. Going forward they will