libchromeos: Added GetMapKeysAsVector() and unit tests

Added chromeos::GetMapKeysAsVector() helper function to obtain
the map as a vector<> instead of a set<>. Also added unit tests
for the map_utils.

BUG=None
TEST=FEATURES=test emerge-link libchromeos buffet

Change-Id: I689a3b0e2c2ddcb532101a5d0750aa789eb2bbda
Reviewed-on: https://chromium-review.googlesource.com/235714
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>

buffet/test_daemon/main.cc

diff --git a/buffet/test_daemon/main.cc b/buffet/test_daemon/main.cc
index ec26a82..4afc35d 100644
--- a/buffet/test_daemon/main.cc
+++ b/buffet/test_daemon/main.cc
@@ -77,9 +77,8 @@
   printf("              ID: %s\n", command->id().c_str());
   printf("          status: %s\n", command->status().c_str());
   printf(" # of parameters: %" PRIuS "\n", command->parameters().size());
-  auto set = chromeos::GetMapKeys(command->parameters());
-  std::string param_names = chromeos::string_utils::Join(
-      ", ", std::vector<std::string>(set.begin(), set.end()));
+  auto keys = chromeos::GetMapKeysAsVector(command->parameters());
+  std::string param_names = chromeos::string_utils::Join(", ", keys);
   printf(" parameter names: %s\n", param_names.c_str());
   OnCommandProgress(command, 0);
 }