[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 <[email protected]>
Reviewed-by: Brian White <[email protected]>
Reviewed-by: Chris Harrelson <[email protected]>
Reviewed-by: Alex Moshchuk <[email protected]>
Cr-Commit-Position: refs/heads/master@{#609039}
diff --git a/third_party/blink/common/features.cc b/third_party/blink/common/features.cc
index dc91fce..53457e69 100644
--- a/third_party/blink/common/features.cc
+++ b/third_party/blink/common/features.cc
@@ -24,6 +24,10 @@
     "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};