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

content/public/common/content_features.cc

Index: content/public/common/content_features.cc
diff --git a/content/public/common/content_features.cc b/content/public/common/content_features.cc
index 9e01f6f97c01a13b3dad443b176be17f7dcbe0bb..e3b3a2587fc4d5c4dd573c5fa9ec3b17c3013f70 100644
--- a/content/public/common/content_features.cc
+++ b/content/public/common/content_features.cc
@@ -19,6 +19,10 @@ const base::Feature kAsmJsToWebAssembly{"AsmJsToWebAssembly",
 const base::Feature kBrotliEncoding{"brotli-encoding",
                                     base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Enables browser side navigation (aka PlzNavigate). http://crbug.com/368813
+const base::Feature kBrowserSideNavigation{"browser-side-navigation",
+                                           base::FEATURE_DISABLED_BY_DEFAULT};
+
 // If Canvas2D Image Chromium is allowed, this feature controls whether it is
 // enabled.
 const base::Feature kCanvas2DImageChromium{"Canvas2DImageChromium",