[Autofill] use vector.emplace_back instead of push_back with pair

Instead of some_vector.push_back(std::make_pair(a, b))
we can use some_vector.emplace_back(a, b)

Bug: 798758
Change-Id: Ib470b5b809a255933706fb03db3cf9ce81fa9fef
Reviewed-on: https://chromium-review.googlesource.com/848485
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526981}

components/autofill/core/browser/region_combobox_model.cc

@@ -87,9 +87,9 @@ void RegionComboboxModel::OnRegionDataLoaded(
   // Some countries expose a state field but have no region names available.
   if (regions.size() > 0) {
     failed_to_load_data_ = false;
-    regions_.push_back(std::make_pair("", "---"));
+    regions_.emplace_back("", "---");
     for (auto* const region : regions) {
-      regions_.push_back(std::make_pair(region->key(), region->name()));
+      regions_.emplace_back(region->key(), region->name());
     }
   } else {
     // TODO(mad): Maybe use a static list as is done for countries in

components/autofill/core/browser/region_combobox_model_unittest.cc

@@ -32,8 +32,8 @@ TEST(RegionComboboxModelTest, QuebecOntarioRegions) {
   model.LoadRegionData("", &test_region_data_loader, 0);
 
   std::vector<std::pair<std::string, std::string>> regions;
-  regions.push_back(std::make_pair(kQuebecCode, kQuebecName));
-  regions.push_back(std::make_pair(kOntarioCode, kOntarioName));
+  regions.emplace_back(kQuebecCode, kQuebecName);
+  regions.emplace_back(kOntarioCode, kOntarioName);
 
   test_region_data_loader.SendAsynchronousData(regions);