Add feature flag for PlzNavigate so that we can run experiments.

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 a94eff16cb802387a9ef9e44530eeb0dc0d947a1..698ce3f3a69450e2d07056ddd4c6f008192b8ebe 100644
--- a/content/public/common/browser_side_navigation_policy.cc
+++ b/content/public/common/browser_side_navigation_policy.cc
@@ -5,13 +5,16 @@
 #include "content/public/common/browser_side_navigation_policy.h"
 
 #include "base/command_line.h"
+#include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 
 namespace content {
 
 bool IsBrowserSideNavigationEnabled() {
-  return base::CommandLine::ForCurrentProcess()->HasSwitch(
-      switches::kEnableBrowserSideNavigation);
+  return
+      base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kEnableBrowserSideNavigation) ||
+      base::FeatureList::IsEnabled(features::kBrowserSideNavigation);
 }
 
 }  // namespace content