Retire PerformancePaintTiming flag

PerformancePaintTiming shipped in M60
https://codereview.chromium.org/2905543002
https://www.chromestatus.com/feature/5688621814251520

BUG=720055

Change-Id: I528c714cb7a913bc9b8e453096aa92ec8eae5e5a
Reviewed-on: https://chromium-review.googlesource.com/1164969Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582052}

third_party/blink/renderer/core/timing/performance.cc

@@ -518,9 +518,6 @@ void Performance::AddFirstContentfulPaintTiming(TimeTicks start_time) {
 
 void Performance::AddPaintTiming(PerformancePaintTiming::PaintType type,
                                  TimeTicks start_time) {
-  if (!RuntimeEnabledFeatures::PerformancePaintTimingEnabled())
-    return;
-
   PerformanceEntry* entry = new PerformancePaintTiming(
       type, MonotonicTimeToDOMHighResTimeStamp(start_time));
   // Always buffer First Paint & First Contentful Paint.

third_party/blink/renderer/core/timing/performance_paint_timing.idl

@@ -4,7 +4,5 @@
 
 // https://w3c.github.io/paint-timing/#sec-PerformancePaintTiming
 
-[
-    RuntimeEnabled=PerformancePaintTiming
-] interface PerformancePaintTiming : PerformanceEntry {
+interface PerformancePaintTiming : PerformanceEntry {
 };

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -930,10 +930,6 @@
       name: "PerformanceNavigationTiming2",
       status: "stable",
     },
-    {
-      name: "PerformancePaintTiming",
-      status: "stable",
-    },
     {
       name: "PermissionDelegation",
       status: "test",