Move ProxyResolver to common.

HttpFetcher depends on it and HttpFetcher is in common.

Bug: 28171891
Test: mma
Change-Id: I1783821c4a7d6785ee23df1cde648a3e1ea8fadf
diff --git a/common/http_fetcher_unittest.cc b/common/http_fetcher_unittest.cc
index 23df67a..66767fb 100644
--- a/common/http_fetcher_unittest.cc
+++ b/common/http_fetcher_unittest.cc
@@ -44,12 +44,12 @@
 #include "update_engine/common/file_fetcher.h"
 #include "update_engine/common/http_common.h"
 #include "update_engine/common/mock_http_fetcher.h"
+#include "update_engine/common/mock_proxy_resolver.h"
 #include "update_engine/common/multi_range_http_fetcher.h"
+#include "update_engine/common/proxy_resolver.h"
 #include "update_engine/common/test_utils.h"
 #include "update_engine/common/utils.h"
 #include "update_engine/libcurl_http_fetcher.h"
-#include "update_engine/mock_proxy_resolver.h"
-#include "update_engine/proxy_resolver.h"
 
 using brillo::MessageLoop;
 using std::make_pair;