Add chrome://flags entry to enable the standalone content suggestions UI.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 0b22cea2132ea04cc6bfb065800416eee850a67e..6ba39501c66ad524f0a2c7334c819b0c2174cd0a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -94707,6 +94707,7 @@ value.
   <int value="0" label="BAD_FLAG_FORMAT">
     Command-line flag doesn't start with two dashes.
   </int>
+  <int value="7444737" label="NTPSuggestionsStandaloneUI:disabled"/>
   <int value="7533886" label="disable-offer-store-unmasked-wallet-cards"/>
   <int value="10458238" label="disable-print-preview-simplify"/>
   <int value="11698808" label="enable-dom-distiller-button-animation"/>
@@ -94784,6 +94785,7 @@ value.
   <int value="411250226" label="AutoplayMutedVideos:disabled"/>
   <int value="412957264" label="tab-close-buttons-hidden-with-touch"/>
   <int value="413081240" label="enable-new-md-input-view"/>
+  <int value="413695227" label="NTPSuggestionsStandaloneUI:enabled"/>
   <int value="415154056" label="enable-physical-keyboard-autocorrect"/>
   <int value="416887895" label="enable-password-change-support"/>
   <int value="422307097" label="PhysicalWeb:disabled"/>