Add benchmark score keys to the hot mode test report. (#12547)

This is follow-up to be1467e0.

dev/devicelab/lib/tasks/hot_mode_tests.dart

@@ -77,7 +77,8 @@ TaskFunction createHotModeTest({ bool isPreviewDart2: false }) {
     });
     final Map<String, dynamic> twoReloadsData = JSON.decode(
       benchmarkFile.readAsStringSync());
-    return new TaskResult.success(<String, dynamic> {
+    return new TaskResult.success(
+      <String, dynamic> {
         'hotReloadInitialDevFSSyncMilliseconds': twoReloadsData['hotReloadInitialDevFSSyncMilliseconds'][0],
         'hotRestartMillisecondsToFrame': twoReloadsData['hotRestartMillisecondsToFrame'][0],
         'hotReloadMillisecondsToFrame' : twoReloadsData['hotReloadMillisecondsToFrame'][0],
@@ -86,7 +87,19 @@ TaskFunction createHotModeTest({ bool isPreviewDart2: false }) {
         'hotReloadVMReloadMilliseconds': twoReloadsData['hotReloadVMReloadMilliseconds'][0],
         'hotReloadDevFSSyncMillisecondsAfterChange': twoReloadsData['hotReloadDevFSSyncMilliseconds'][1],
         'hotReloadFlutterReassembleMillisecondsAfterChange': twoReloadsData['hotReloadFlutterReassembleMilliseconds'][1],
-      'hotReloadVMReloadMillisecondsAfterChange': twoReloadsData['hotReloadVMReloadMilliseconds'][1]
-    });
+        'hotReloadVMReloadMillisecondsAfterChange': twoReloadsData['hotReloadVMReloadMilliseconds'][1],
+      },
+      benchmarkScoreKeys: <String>[
+        'hotReloadInitialDevFSSyncMilliseconds',
+        'hotRestartMillisecondsToFrame',
+        'hotReloadMillisecondsToFrame',
+        'hotReloadDevFSSyncMilliseconds',
+        'hotReloadFlutterReassembleMilliseconds',
+        'hotReloadVMReloadMilliseconds',
+        'hotReloadDevFSSyncMillisecondsAfterChange',
+        'hotReloadFlutterReassembleMillisecondsAfterChange',
+        'hotReloadVMReloadMillisecondsAfterChange',
+      ]
+    );
   };
 }