Upgrade ash/system/power from deprecated base::JSONReader API

Bug: 1070409
Change-Id: I2e275aeda6c16e3f9b02031be26fee5967e8f549
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228273Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775054}

ash/system/power/power_prefs_unittest.cc

@@ -169,11 +169,12 @@ std::string GetExpectedAdvancedBatteryChargeModePolicyForPrefs(
 
 void DecodeJsonStringAndNormalize(const std::string& json_string,
                                   base::Value* value) {
-  base::JSONReader reader(base::JSON_ALLOW_TRAILING_COMMAS);
-  base::Optional<base::Value> read_value = reader.ReadToValue(json_string);
-  ASSERT_EQ(reader.GetErrorMessage(), "");
-  ASSERT_TRUE(read_value.has_value());
-  *value = std::move(read_value.value());
+  base::JSONReader::ValueWithError parsed_json =
+      base::JSONReader::ReadAndReturnValueWithError(
+          json_string, base::JSON_ALLOW_TRAILING_COMMAS);
+  ASSERT_EQ(parsed_json.error_message, "");
+  ASSERT_TRUE(parsed_json.value);
+  *value = std::move(*parsed_json.value);
 }
 
 }  // namespace