update_engine: Replace googe::protobuf::Closure with base::Closure

In order to clean up libchromeos, remove chromeos/callback.h and
switch update_engine from using
googe::protobuf::Closure/NewPermanentCallback to base::Closure/Bind

BUG=chromium:406060
TEST=FEATURES=test emerge-link update_engine

Change-Id: I02d222c3f6c9a5bf5177d58e3a25a596348619bf
Reviewed-on: https://chromium-review.googlesource.com/213675
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/http_fetcher.cc b/http_fetcher.cc
index 2dd6ec4..7fa2812 100644
--- a/http_fetcher.cc
+++ b/http_fetcher.cc
@@ -4,7 +4,7 @@
 
 #include "update_engine/http_fetcher.h"
 
-using google::protobuf::Closure;
+using base::Closure;
 using std::deque;
 using std::string;