Report total_ui_gc_time to SkiaPerf (#125919)

Not sure why this wasn't already being reported.

dev/devicelab/lib/tasks/perf_tests.dart

@@ -936,6 +936,7 @@ class PerfTest {
     this.timelineFileName, {
     this.measureCpuGpu = true,
     this.measureMemory = true,
+    this.measureTotalGCTime = true,
     this.saveTraceFile = false,
     this.testDriver,
     this.needsFullTimeline = true,
@@ -953,6 +954,7 @@ class PerfTest {
     this.testTarget, {
     this.measureCpuGpu = false,
     this.measureMemory = false,
+    this.measureTotalGCTime = false,
     this.testDriver =  'test_driver/e2e_test.dart',
     this.needsFullTimeline = false,
     this.benchmarkScoreKeys = _kCommonScoreKeys,
@@ -979,6 +981,8 @@ class PerfTest {
   final bool measureCpuGpu;
   /// Whether to collect memory metrics.
   final bool measureMemory;
+  /// Whether to summarize total GC time on the UI thread from the timeline.
+  final bool measureTotalGCTime;
   /// Whether to collect full timeline, meaning if `--trace-startup` flag is needed.
   final bool needsFullTimeline;
   /// Whether to save the trace timeline file `*.timeline.json`.
@@ -1114,6 +1118,7 @@ class PerfTest {
             if (data['90th_percentile_memory_usage'] != null) '90th_percentile_memory_usage',
             if (data['99th_percentile_memory_usage'] != null) '99th_percentile_memory_usage',
           ],
+          if (measureTotalGCTime) 'total_ui_gc_time',
           if (data['30hz_frame_percentage'] != null) '30hz_frame_percentage',
           if (data['60hz_frame_percentage'] != null) '60hz_frame_percentage',
           if (data['80hz_frame_percentage'] != null) '80hz_frame_percentage',

dev/devicelab/test/perf_tests_test.dart

@@ -52,6 +52,7 @@ void main() {
       '90th_percentile_picture_cache_memory': 1,
       '99th_percentile_picture_cache_memory': 1,
       'worst_picture_cache_memory': 1,
+      'total_ui_gc_time': 1,
       'new_gen_gc_count': 1,
       'old_gen_gc_count': 1,
       'average_vsync_transitions_missed': 1,
@@ -95,6 +96,7 @@ void main() {
       '90th_percentile_picture_cache_memory': 1,
       '99th_percentile_picture_cache_memory': 1,
       'worst_picture_cache_memory': 1,
+      'total_ui_gc_time': 1,
       'new_gen_gc_count': 1,
       'old_gen_gc_count': 1,
       'average_vsync_transitions_missed': 1,