Flip PageLifecycle flag to experimental.

I2I link:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/1WFVPbRL640/TPPG9gs5AwAJ

Bug: 809283
Change-Id: Id0942f146cffc07981b135ca0590a7fab1b23140
Reviewed-on: https://chromium-review.googlesource.com/903183Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Reviewed-by: Ojan Vafai <ojan@chromium.org>
Commit-Queue: Shubhie Panicker <panicker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534757}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -766,7 +766,7 @@
     },
     {
       name: "PageLifecycle",
-      status: "test",
+      status: "experimental",
     },
     {
       name: "PagePopup",