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 4a18b4cce14806409753ac880915a3d9dbc0a375..6d7d2dc9847affce3d0b5e642bc3aa4798fcd1b9 100644
--- a/chrome/browser/android/webapk/webapk.proto
+++ b/chrome/browser/android/webapk/webapk.proto
@@ -45,6 +45,11 @@ 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 avaialbe. Therefore, an empty WebApkManifest is sent to the
+  // server.
+  optional bool stale_manifest = 9;
+
   reserved 4;
 }