Add a feature flag for the new Windows USB backend.

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 8b3802a0819af5213e3a5f05be526f5428d41900..b23b67321ff4bc2b96b77f15fa2155a69ff540a8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -96767,6 +96767,7 @@ value.
   <int value="-1491417046" label="enable-fullscreen-toolbar-reveal"/>
   <int value="-1490298774" label="enable-captive-portal-bypass-proxy-option"/>
   <int value="-1488744539" label="QuickUnlockFingerprint:enabled"/>
+  <int value="-1487243228" label="NewUsbBackend:disabled"/>
   <int value="-1482685863" label="enable-request-tablet-site"/>
   <int value="-1480926949" label="MaterialDesignBookmarks:enabled"/>
   <int value="-1478876902" label="disable-permission-action-reporting"/>
@@ -97149,6 +97150,7 @@ value.
   <int value="278756320" label="disable-app-list-app-info"/>
   <int value="280644887" label="mash"/>
   <int value="301869874" label="NTPPhysicalWebPageSuggestions:disabled"/>
+  <int value="304901781" label="NewUsbBackend:enabled"/>
   <int value="313303258" label="WebPaymentsModifiers:disabled"/>
   <int value="316182183" label="MediaDocumentDownloadButton:disabled"/>
   <int value="323605372" label="ui-disable-compositor-animation-timelines"/>