[Passwords] Simplify CSVPasswordSequence

This change simplifies the NameToLabel helper inside CSVPasswordSequence
to use the new base::fixed_flat_map and return a simple pointer instead
of a base::Optional.

Bug: 934326
Change-Id: I1325a7470151ee31af37dcc21919d696d829edbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2544606Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828807}

components/password_manager/core/browser/import/csv_password_sequence.cc

@@ -8,10 +8,9 @@
 #include <string>
 #include <utility>
 
-#include "base/containers/flat_map.h"
+#include "base/containers/fixed_flat_map.h"
 #include "base/containers/flat_set.h"
-#include "base/no_destructor.h"
-#include "base/optional.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "components/password_manager/core/browser/import/csv_field_parser.h"
 #include "components/password_manager/core/browser/import/csv_password.h"
@@ -21,13 +20,13 @@ namespace password_manager {
 
 namespace {
 
-// Given a CSV column |name|, returns the matching CSVPassword::Label or nothing
-// if the column name is not recognised.
-base::Optional<CSVPassword::Label> NameToLabel(base::StringPiece name) {
+// Given a CSV column |name|, returns a pointer to the matching
+// CSVPassword::Label or nullptr if the column name is not recognised.
+const CSVPassword::Label* NameToLabel(base::StringPiece name) {
   using Label = CSVPassword::Label;
   // Recognised column names for origin URL, usernames and passwords.
-  static const base::NoDestructor<base::flat_map<base::StringPiece, Label>>
-      kLabelMap({
+  static constexpr auto kLabelMap =
+      base::MakeFixedFlatMap<base::StringPiece, Label>({
           {"url", Label::kOrigin},
           {"website", Label::kOrigin},
           {"origin", Label::kOrigin},
@@ -40,9 +39,8 @@ base::Optional<CSVPassword::Label> NameToLabel(base::StringPiece name) {
 
           {"password", Label::kPassword},
       });
-  auto it = kLabelMap->find(base::ToLowerASCII(name));
-  return it != kLabelMap->end() ? base::make_optional(it->second)
-                                : base::nullopt;
+  auto* it = kLabelMap.find(base::ToLowerASCII(name));
+  return it != kLabelMap.end() ? &it->second : nullptr;
 }
 
 }  // namespace
@@ -65,10 +63,9 @@ CSVPasswordSequence::CSVPasswordSequence(std::string csv)
       result_ = CSVPassword::Status::kSyntaxError;
       return;
     }
-    base::Optional<CSVPassword::Label> label = NameToLabel(name);
-    if (label) {
+
+    if (const CSVPassword::Label* label = NameToLabel(name))
       map_[col_index] = *label;
-    }
   }
 
   // Check that each of the three labels is assigned to exactly one column.