Add comment to explain why padding is removed.

Change-Id: I0882e0c5d99310b179ace493b937669520626263

Author: huangjiahua

cab-token-generator/java/com/google/auth/credentialaccessboundary/ClientSideCredentialAccessBoundaryFactory.java

@@ -206,6 +206,7 @@ public AccessToken generateToken(CredentialAccessBoundary accessBoundary)
 
     byte[] encryptedRestrictions = this.encryptRestrictions(rawRestrictions, sessionKey);
 
+    // `withoutPadding` is used to stay consistent with server-side CAB
     String tokenValue =
         intermediateToken + "." + Base64.getUrlEncoder().withoutPadding().encodeToString(encryptedRestrictions);