Add a flag in WebAPK's proto when the Web App Manifest is no longer available.

chrome/browser/android/webapk/webapk.proto

Index: chrome/browser/android/webapk/webapk.proto
diff --git a/chrome/browser/android/webapk/webapk.proto b/chrome/browser/android/webapk/webapk.proto
index 90b171ed70149e57106955a99317a96aa7f74fe8..05615266b6673f476a9457578ac0f929d1ffcd51 100644
--- a/chrome/browser/android/webapk/webapk.proto
+++ b/chrome/browser/android/webapk/webapk.proto
@@ -48,6 +48,10 @@ message WebApk {
   // The cpu abi of the browser making the request.
   optional string android_abi = 8;
 
+  // If set true, this flag indicates that the Web App Manifest of the site is
+  // no longer available.
+  optional bool stale_manifest = 9;
+
   reserved 4;
 }