update_engine: Fix build breaks in unit tests on Chrome OS

Some unit tests that compile only on Chrome OS were broken.

Change-Id: I47b27ec49f11137e7549da0b7a8d2d6d9fef768c
diff --git a/fake_p2p_manager_configuration.h b/fake_p2p_manager_configuration.h
index 3d3afe4..2f05ba3 100644
--- a/fake_p2p_manager_configuration.h
+++ b/fake_p2p_manager_configuration.h
@@ -63,8 +63,9 @@
     // Replace {variable} on the passed string.
     std::string str_minimum_size = std::to_string(minimum_size);
     for (std::string& arg : formatted_command) {
-      ReplaceSubstringsAfterOffset(&arg, 0, "{file_id}", file_id);
-      ReplaceSubstringsAfterOffset(&arg, 0, "{minsize}", str_minimum_size);
+      base::ReplaceSubstringsAfterOffset(&arg, 0, "{file_id}", file_id);
+      base::ReplaceSubstringsAfterOffset(&arg, 0, "{minsize}",
+                                         str_minimum_size);
     }
     return formatted_command;
   }