Fix unittest key path in Brillo.

The unittests weren't able to find the keys if it's not run from the
update_engine_unittests directory.

Test: /data/nativetest/update_engine_unittests/update_engine_unittests
Bug: 26955860

Change-Id: I4c189cca2714986d3d2b2669b54a5dff9a488964
diff --git a/common/http_fetcher_unittest.cc b/common/http_fetcher_unittest.cc
index 0d4b5da..e287103 100644
--- a/common/http_fetcher_unittest.cc
+++ b/common/http_fetcher_unittest.cc
@@ -117,9 +117,7 @@
 
     // Spawn the server process.
     unique_ptr<brillo::Process> http_server(new brillo::ProcessImpl());
-    base::FilePath test_server_path =
-        test_utils::GetBuildArtifactsPath().Append("test_http_server");
-    http_server->AddArg(test_server_path.value());
+    http_server->AddArg(test_utils::GetBuildArtifactsPath("test_http_server"));
     http_server->RedirectUsingPipe(STDOUT_FILENO, false);
 
     if (!http_server->Start()) {
diff --git a/common/test_utils.cc b/common/test_utils.cc
index 2de8ced..13ce6f9 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -263,5 +263,9 @@
   return exe_path.DirName();
 }
 
+string GetBuildArtifactsPath(const string& relative_path) {
+  return GetBuildArtifactsPath().Append(relative_path).value();
+}
+
 }  // namespace test_utils
 }  // namespace chromeos_update_engine
diff --git a/common/test_utils.h b/common/test_utils.h
index e4919d8..952cbea 100644
--- a/common/test_utils.h
+++ b/common/test_utils.h
@@ -190,6 +190,8 @@
 // Returns the path where the build artifacts are stored. This is the directory
 // where the unittest executable is being run from.
 base::FilePath GetBuildArtifactsPath();
+// Returns the path of the build artifact specified in |relative_path|.
+std::string GetBuildArtifactsPath(const std::string& relative_path);
 
 }  // namespace test_utils