[FCP++] Enable FCP++ by default in web-test

Currently FCP++ is not enabled during web-test because the feature_list
override the config of runtime_enabled_features. As FCP++ has now been enabled
by default, we should change this config of feature_list as well.

Bug:966216

Change-Id: I8785b6c31cf6fe7af3f9662c3af50262cd137977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645897Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667277}

testing/variations/fieldtrial_testing_config.json

@@ -2225,32 +2225,6 @@
             ]
         }
     ],
-    "FirstContentfulPaintPlusPlus": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows",
-                "ios"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "FirstContentfulPaintPlusPlus"
-                    ]
-                },
-                {
-                    "name": "Disabled",
-                    "disable_features": [
-                        "FirstContentfulPaintPlusPlus"
-                    ]
-                }
-            ]
-        }
-    ],
     "ForbidSyncXHRInPageDismissal": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -46,7 +46,7 @@ const base::Feature kUserLevelMemoryPressureSignal{
 
 // Enable FCP++ by experiment. See https://crbug.com/869924
 const base::Feature kFirstContentfulPaintPlusPlus{
-    "FirstContentfulPaintPlusPlus", base::FEATURE_DISABLED_BY_DEFAULT};
+    "FirstContentfulPaintPlusPlus", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Perform memory purges after freezing only if all pages are frozen.
 const base::Feature kFreezePurgeMemoryAllPagesFrozen{

third_party/blink/renderer/core/paint/image_paint_timing_detector.cc

@@ -207,7 +207,7 @@ void ImagePaintTimingDetector::ReportSwapTime(
 void ImageRecordsManager::AssignPaintTimeToRegisteredQueuedNodes(
     const base::TimeTicks& timestamp,
     unsigned last_queued_frame_index) {
-  DCHECK(!images_queued_for_paint_time_.empty());
+  // TODO(crbug.com/971419): should guarantee the queue not empty.
   while (!images_queued_for_paint_time_.empty()) {
     base::WeakPtr<ImageRecord>& record = images_queued_for_paint_time_.front();
     if (record->frame_index > last_queued_frame_index)