Add update_engine_host_unittests

Some of update_engine's codebase (payload generator stuff) run on host.
So it make sense to test them on the host. Adding a new unittest target
update_engine_host_unittests, which includes a subset of unittest. After
this CL, the old update_engine_unittests target remains unchanged, it
still contains all 600 unittests. The new host target will contain a
subset of these.

Test: th
Change-Id: I49c8310cee9a2b2ea197b8ab60ea3b00bf5680cf
diff --git a/common/file_fetcher.h b/common/file_fetcher.h
index bd39007..0f034e3 100644
--- a/common/file_fetcher.h
+++ b/common/file_fetcher.h
@@ -23,7 +23,6 @@
 
 #include <base/logging.h>
 #include <base/macros.h>
-#include <brillo/message_loops/message_loop.h>
 #include <brillo/streams/stream.h>
 
 #include "update_engine/common/http_fetcher.h"
diff --git a/common/http_fetcher.h b/common/http_fetcher.h
index 7fa5f09..80985af 100644
--- a/common/http_fetcher.h
+++ b/common/http_fetcher.h
@@ -170,7 +170,7 @@
   ErrorCode auxiliary_error_code_{ErrorCode::kSuccess};
 
   // The delegate; may be null.
-  HttpFetcherDelegate* delegate_;
+  HttpFetcherDelegate* delegate_ = nullptr;
 
   // Proxy servers
   std::deque<std::string> proxies_;