Add a feature flag for network service and about:flags entry for it.

content/public/common/browser_side_navigation_policy.cc

Index: content/public/common/browser_side_navigation_policy.cc
diff --git a/content/public/common/browser_side_navigation_policy.cc b/content/public/common/browser_side_navigation_policy.cc
index 698ce3f3a69450e2d07056ddd4c6f008192b8ebe..67f2ccb9c74224eaa2fae1c7cf34ea057b4067df 100644
--- a/content/public/common/browser_side_navigation_policy.cc
+++ b/content/public/common/browser_side_navigation_policy.cc
@@ -11,10 +11,10 @@
 namespace content {
 
 bool IsBrowserSideNavigationEnabled() {
-  return
-      base::CommandLine::ForCurrentProcess()->HasSwitch(
-          switches::kEnableBrowserSideNavigation) ||
-      base::FeatureList::IsEnabled(features::kBrowserSideNavigation);
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+             switches::kEnableBrowserSideNavigation) ||
+         base::FeatureList::IsEnabled(features::kBrowserSideNavigation) ||
+         base::FeatureList::IsEnabled(features::kNetworkService);
 }
 
 }  // namespace content