Use base::flat_[set|map] over std::[set|map] in Print Preview UI

For small maps and sets that contain efficiently moveable items,
base::flat_[set|map] performs inserts better than std::[set|map],
because the former avoids mallocs. Additionally, base::flat_[set|map]
has a smaller overhead than its STL counterpart. Usage recommendation
can be found at base/containers/README.md.

All the substitutions in this CL are made on containers that are
expected to be small (< dozens of entries) and that hold light and
easily moveable items.

Change-Id: I2fe35182cd7c85479db4b7b6343ea9af031031ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925438
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718312}

chrome/browser/ui/webui/print_preview/print_preview_handler.cc

@@ -16,7 +16,6 @@
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
-#include "base/containers/flat_map.h"
 #include "base/feature_list.h"
 #include "base/i18n/number_formatting.h"
 #include "base/json/json_reader.h"

chrome/browser/ui/webui/print_preview/print_preview_handler.h

@@ -5,11 +5,11 @@
 #ifndef CHROME_BROWSER_UI_WEBUI_PRINT_PREVIEW_PRINT_PREVIEW_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_PRINT_PREVIEW_PRINT_PREVIEW_HANDLER_H_
 
-#include <map>
 #include <memory>
-#include <set>
 #include <string>
 
+#include "base/containers/flat_map.h"
+#include "base/containers/flat_set.h"
 #include "base/files/file_path.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
@@ -353,13 +353,13 @@ class PrintPreviewHandler : public content::WebUIMessageHandler,
   std::unique_ptr<PrinterHandler> local_printer_handler_;
 
   // Maps preview request ids to callbacks.
-  std::map<int, std::string> preview_callbacks_;
+  base::flat_map<int, std::string> preview_callbacks_;
 
   // Set of preview request ids for failed previews.
-  std::set<int> preview_failures_;
+  base::flat_set<int> preview_failures_;
 
   // Set of printer types on the deny list.
-  std::set<PrinterType> printer_type_deny_list_;
+  base::flat_set<PrinterType> printer_type_deny_list_;
 
   base::WeakPtrFactory<PrintPreviewHandler> weak_factory_{this};
 

chrome/browser/ui/webui/print_preview/print_preview_ui.cc

@@ -4,12 +4,12 @@
 
 #include "chrome/browser/ui/webui/print_preview/print_preview_ui.h"
 
-#include <map>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
+#include "base/containers/flat_map.h"
 #include "base/containers/id_map.h"
 #include "base/feature_list.h"
 #include "base/lazy_instance.h"
@@ -90,7 +90,7 @@ constexpr char kGeneratedPath[] =
 
 PrintPreviewUI::TestDelegate* g_test_delegate = nullptr;
 
-// Thread-safe wrapper around a std::map to keep track of mappings from
+// Thread-safe wrapper around a base::flat_map to keep track of mappings from
 // PrintPreviewUI IDs to most recent print preview request IDs.
 class PrintPreviewRequestIdMapWithLock {
  public:
@@ -122,7 +122,7 @@ class PrintPreviewRequestIdMapWithLock {
 
  private:
   // Mapping from PrintPreviewUI ID to print preview request ID.
-  typedef std::map<int, int> PrintPreviewRequestIdMap;
+  using PrintPreviewRequestIdMap = base::flat_map<int, int>;
 
   PrintPreviewRequestIdMap map_;
   base::Lock lock_;