Retire PerformanceNavigationTiming2 runtime enabled flag.

Feature first shipped in M57.

BUG=611276

Change-Id: Ib4b514df4f72b29333b534e6978ea8fa3dea5128
Reviewed-on: https://chromium-review.googlesource.com/c/1323628Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606871}

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

@@ -11,7 +11,6 @@ enum NavigationType {
     "prerender"
 };
 [
-    RuntimeEnabled=PerformanceNavigationTiming2,
     Exposed=Window
 ]  interface PerformanceNavigationTiming : PerformanceResourceTiming {
     readonly attribute DOMHighResTimeStamp unloadEventStart;

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

@@ -178,8 +178,6 @@ bool WindowPerformance::shouldYield() const {
 
 PerformanceNavigationTiming*
 WindowPerformance::CreateNavigationTimingInstance() {
-  if (!RuntimeEnabledFeatures::PerformanceNavigationTiming2Enabled())
-    return nullptr;
   if (!GetFrame())
     return nullptr;
   const DocumentLoader* document_loader =

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -938,10 +938,6 @@
       name: "PaymentRetry",
       status: "experimental",
     },
-    {
-      name: "PerformanceNavigationTiming2",
-      status: "stable",
-    },
     {
       name: "PermissionDelegation",
       status: "test",