update_engine: Adds back a unittest

Adds back unittest chrome_browser_proxy_resolver_unittest.cc which was
accidentally removed in CL:636104

BUG=none
TEST=cros_workon_make --board=amd64-generic --test update_engine

Change-Id: Ia638e43d47f45f069a3f7206d9b6280eb9d33799
Reviewed-on: https://chromium-review.googlesource.com/638650
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/update_engine.gyp b/update_engine.gyp
index b40a23f..c828a49 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -574,6 +574,13 @@
             'update_manager/update_manager_unittest.cc',
             'update_manager/variable_unittest.cc',
           ],
+          'conditions': [
+            ['USE_chrome_network_proxy == 1', {
+              'sources': [
+                'chrome_browser_proxy_resolver_unittest.cc',
+              ],
+            }],
+          ],
         },
       ],
     }],