Add DelayNavigationThrottle

DelayNavigationThrottle delays all HTTP/HTTPS main frame navigations
by an amount specified in a field trial parameter.

BUG=704117

Review-Url: https://codereview.chromium.org/2763613002
Cr-Commit-Position: refs/heads/master@{#459575}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 640e7ce..f2ff140 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -780,6 +780,8 @@
     "offline_items_collection/offline_content_aggregator_factory.h",
     "page_load_metrics/browser_page_track_decider.cc",
     "page_load_metrics/browser_page_track_decider.h",
+    "page_load_metrics/experiments/delay_navigation_throttle.cc",
+    "page_load_metrics/experiments/delay_navigation_throttle.h",
     "page_load_metrics/metrics_navigation_throttle.cc",
     "page_load_metrics/metrics_navigation_throttle.h",
     "page_load_metrics/metrics_web_contents_observer.cc",