Don't download identical APKs on update.

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 05615266b6673f476a9457578ac0f929d1ffcd51..ce003361ae1563a8413344fb5f3336488e51cf57 100644
--- a/chrome/browser/android/webapk/webapk.proto
+++ b/chrome/browser/android/webapk/webapk.proto
@@ -16,7 +16,12 @@ message WebApkResponse {
   // Version code of the WebAPK.
   optional string version = 2;
 
-  // URL to download WebAPK.
+  // URL to download WebAPK. Note that this can be empty, and if it is, the
+  // client should not attempt to download or install anything. For example,
+  // this may be empty when the client requests an update even though no content
+  // has changed. The WAM server can then let the client know that it is
+  // up-to-date already and does not need to spend bandwidth re-downloading a
+  // WebAPK it already has.
   optional string signed_download_url = 3;
 
   // Unique id identifying session with WebAPK server.