Add chrome://flag to enable the Stale Previews timestamp

chrome/browser/flag_descriptions.cc

Index: chrome/browser/flag_descriptions.cc
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index c9876e023206f47c212f43ecd78c21ec37fe3d62..403e59ba085e1a704970400c4395d916f35f2ed5 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -1195,6 +1195,12 @@ const char kEnableDataReductionProxySavingsPromoDescription[] =
     "already saved 1 MB of data, then the promo will not be shown. Data "
     "Saver must be enabled for the promo to be shown.";
 
+const char kEnableStalePreviewsName[] = "Stale Previews Timestamp";
+
+const char kEnableStalePreviewsDescription[] =
+    "Enable showing a timestamp that says how old a Preview is on the Previews "
+    "infobar.";
+
 #if defined(OS_ANDROID)
 
 const char kEnableDataReductionProxyMainMenuName[] =