ui + mus: Split ContextFactory into ContextFactory(Client) and ContextFactoryPrivate

ui/aura/env.cc

Index: ui/aura/env.cc
diff --git a/ui/aura/env.cc b/ui/aura/env.cc
index d4b5cdfe43d0d7914753efd2975cba71acaa66db..5e9411332e3aacd7299bd6c5c1a08fc6c58e8fce 100644
--- a/ui/aura/env.cc
+++ b/ui/aura/env.cc
@@ -156,7 +156,8 @@ Env::Env(Mode mode)
       mouse_button_flags_(0),
       is_touch_down_(false),
       input_state_lookup_(InputStateLookup::Create()),
-      context_factory_(NULL) {
+      context_factory_(nullptr),
+      context_factory_private_(nullptr) {
   DCHECK(lazy_tls_ptr.Pointer()->Get() == NULL);
   lazy_tls_ptr.Pointer()->Set(this);
 }