Convert the overlay scrollbar flag to use FeatureList

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 5c644c89bdedc7494b375c9e31d5ee583f8da387..267b964460c3a0f4673874bb3f2ea08b21144104 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -103540,6 +103540,7 @@ from previous Chrome versions.
   <int value="1694854500" label="disable-save-password-bubble"/>
   <int value="1696139514" label="enable-ble-advertising-in-apps"/>
   <int value="1697189972" label="WebPaymentsSingleAppUiSkip:disabled"/>
+  <int value="1700394127" label="OverlayScrollbar:disabled"/>
   <int value="1701972870" label="NTPSnippetsIncreasedVisibility:enabled"/>
   <int value="1702821235" label="WebAssembly:enabled"/>
   <int value="1705724232" label="use-android-midi-api"/>
@@ -103600,6 +103601,7 @@ from previous Chrome versions.
   <int value="1900529524" label="disable-touch-drag-drop"/>
   <int value="1905465678" label="ContextualSearchSingleActions:enabled"/>
   <int value="1906942630" label="enable-easy-unlock"/>
+  <int value="1913298816" label="OverlayScrollbar:enabled"/>
   <int value="1915178511" label="disable-blink-features"/>
   <int value="1927259098" label="TranslateLanguageByULP:enabled"/>
   <int value="1928407249" label="NewPhotoPicker:enabled"/>