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 11f243c887a498ed580e0ea96e788ccf7158ebe2..cf36b67f79d5bf5804cbb0829339945c25e4cc2d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -94932,6 +94932,7 @@ 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"/>
@@ -95009,6 +95010,7 @@ 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"/>