Flip nav timing 2 runtime flag to experimental.

This is to gather early feedback in canary, before we flip this to stable (for M57).

Link to Intent to ship.
https://groups.google.com/a/chromium.org/forum/#!topic/blink-
dev/AT7aqhPcNdw

BUG=672982

Review-Url: https://codereview.chromium.org/2560313002
Cr-Commit-Position: refs/heads/master@{#438013}

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -276,5 +276,5 @@ Worklet status=experimental
 LazyParseCSS status=experimental
 ParseHTMLOnMainThread status=test
 SendBeaconThrowForBlobWithNonSimpleType status=experimental
-PerformanceNavigationTiming2 status=test
+PerformanceNavigationTiming2 status=experimental
 BackgroundVideoTrackOptimization status=stable