Add about:flags entry for Mac V2 sandbox.

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index 2de515aeb0fa073b281477a97865fb659be0b89a..f3f1fc12f098b8928a26ddd416d812365e8ba47c 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -22278,6 +22278,7 @@ from previous Chrome versions.
   <int value="-1212273428" label="enable-experimental-app-list"/>
   <int value="-1212167260" label="disable-app-window-cycling"/>
   <int value="-1208501269" label="AutofillScanThemeDialog:enabled"/>
+  <int value="-1206698676" label="MacV2Sandbox:disabled"/>
   <int value="-1206337150"
       label="OmniboxUIExperimentHideSuggestionUrlScheme:disabled"/>
   <int value="-1203955801" label="enable-password-change-support:disabled"/>
@@ -23116,6 +23117,7 @@ from previous Chrome versions.
   <int value="1785093465" label="enable-document-passive-event-listeners"/>
   <int value="1786229999" label="disable-md-downloads"/>
   <int value="1786386775" label="TranslateCompactUI:disabled"/>
+  <int value="1789517771" label="MacV2Sandbox:enabled"/>
   <int value="1803465156" label="enable-zero-suggest-most-visited"/>
   <int value="1809940714" label="SpeculativeLaunchServiceWorker:disabled"/>
   <int value="1812368073" label="enable-new-app-list-mixer"/>