[FCP++] Enable FCP++ metrics via finch experiment

Create a Finch experiment to turn on FCP++ metrics.

We will monitor the heart beat metrics to guarantee that FCP++ won't
introduce regressions.

Bug: 869924
Change-Id: I68b7e446c4e49bc173dc1eccf6e88a9284f90bcd
Reviewed-on: https://chromium-review.googlesource.com/c/1336555
Commit-Queue: Liquan (Max) Gǔ <maxlg@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609039}

content/child/runtime_features.cc

@@ -513,6 +513,10 @@ void SetRuntimeFeaturesDefaultsAndUpdateFromArgs(
       base::FeatureList::IsEnabled(blink::features::kJankTracking) ||
       enableExperimentalWebPlatformFeatures);
 
+  WebRuntimeFeatures::EnableFirstContentfulPaintPlusPlus(
+      base::FeatureList::IsEnabled(
+          blink::features::kFirstContentfulPaintPlusPlus));
+
   WebRuntimeFeatures::EnableNoHoverDuringScroll(
       base::FeatureList::IsEnabled(features::kNoHoverDuringScroll));
 }

testing/variations/fieldtrial_testing_config.json

@@ -2025,6 +2025,32 @@
             ]
         }
     ],
+    "FirstContentfulPaintPlusPlus": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows",
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "FirstContentfulPaintPlusPlus"
+                    ]
+                },
+                {
+                    "name": "Disabled",
+                    "disable_features": [
+                        "FirstContentfulPaintPlusPlus"
+                    ]
+                }
+            ]
+        }
+    ],
     "FrameTypePriorityExperiment": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -24,6 +24,10 @@ const base::Feature kEnableGpuRasterizationViewportRestriction{
     "EnableGpuRasterizationViewportRestriction",
     base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Enable FCP++ by experiment. See https://crbug.com/869924
+const base::Feature kFirstContentfulPaintPlusPlus{
+    "FirstContentfulPaintPlusPlus", base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Enable LayoutNG.
 const base::Feature kLayoutNG{"LayoutNG", base::FEATURE_DISABLED_BY_DEFAULT};
 

third_party/blink/public/common/features.h

@@ -16,6 +16,7 @@ BLINK_COMMON_EXPORT extern const base::Feature
     kEagerCacheStorageSetupForServiceWorkers;
 BLINK_COMMON_EXPORT extern const base::Feature
     kEnableGpuRasterizationViewportRestriction;
+BLINK_COMMON_EXPORT extern const base::Feature kFirstContentfulPaintPlusPlus;
 BLINK_COMMON_EXPORT extern const base::Feature kLayoutNG;
 BLINK_COMMON_EXPORT extern const base::Feature kMojoBlobURLs;
 BLINK_COMMON_EXPORT extern const base::Feature kOnionSoupDOMStorage;

third_party/blink/public/platform/web_runtime_features.h

@@ -100,6 +100,7 @@ class WebRuntimeFeatures {
   BLINK_PLATFORM_EXPORT static void EnableDisplayCutoutAPI(bool);
   BLINK_PLATFORM_EXPORT static void EnableFastMobileScrolling(bool);
   BLINK_PLATFORM_EXPORT static void EnableFileSystem(bool);
+  BLINK_PLATFORM_EXPORT static void EnableFirstContentfulPaintPlusPlus(bool);
   BLINK_PLATFORM_EXPORT static void EnableForceTallerSelectPopup(bool);
   BLINK_PLATFORM_EXPORT static void EnableGamepadExtensions(bool);
   BLINK_PLATFORM_EXPORT static void EnableGamepadVibration(bool);

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

@@ -282,9 +282,8 @@ void ImagePaintTimingDetector::ReportSwapTime(
     base::TimeTicks timestamp) {
   // The callback is safe from race-condition only when running on main-thread.
   DCHECK(ThreadState::Current()->IsMainThread());
-  // This callback is queued only when there are records in
-  // records_pending_timing_.
-  DCHECK_GT(records_pending_timing_.size(), 0UL);
+  // Not guranteed to be non-empty, because record can be removed before the
+  // callback.
   while (records_pending_timing_.size() > 0) {
     DOMNodeId node_id = records_pending_timing_.front();
     if (!id_record_map_.Contains(node_id)) {

third_party/blink/renderer/platform/exported/web_runtime_features.cc

@@ -160,6 +160,10 @@ void WebRuntimeFeatures::EnableFileSystem(bool enable) {
   RuntimeEnabledFeatures::SetFileSystemEnabled(enable);
 }
 
+void WebRuntimeFeatures::EnableFirstContentfulPaintPlusPlus(bool enable) {
+  RuntimeEnabledFeatures::SetFirstContentfulPaintPlusPlusEnabled(enable);
+}
+
 void WebRuntimeFeatures::EnableForceTallerSelectPopup(bool enable) {
   RuntimeEnabledFeatures::SetForceTallerSelectPopupEnabled(enable);
 }