Re-design Unsubscribe and getPermissionStatus mojo interfaces.

content/common/push_messaging.mojom

Index: content/common/push_messaging.mojom
diff --git a/content/common/push_messaging.mojom b/content/common/push_messaging.mojom
index ae843a2834682583c91d2a39be83a71068be1612..f4d83c27f0dc1b0f7970b24e4392f9dd643ffac0 100644
--- a/content/common/push_messaging.mojom
+++ b/content/common/push_messaging.mojom
@@ -134,12 +134,12 @@ interface PushMessaging {
                array<uint8>? p256dh,
                array<uint8>? auth);
 
-  // It returns |did_unsubscribe| if is_success is true, and returns
-  // |error_type| and |error_message| if is_success is false.
+  // We use the value of |error_type| as a flag. If |error_type| == Unknown, it
+  // means no error and returns |did_unsubscribe|. Else, it means there is an
+  // error and returns |error_type| and |error_message|.

[Peter Beverloo, 2017/02/24 15:55:35]

What about renaming PushErrorType::UNKNOWN to PushErrorType::NONE? That'll
remove the need for pretty much all clarifying comments you're adding.

It also means we can do a minor clean-up in:
  third_party/WebKit/Source/modules/push_messaging/PushError.cpp


By changing the WebPushError::ErrorTypeUnknown case to be NOTREACHED() as well.
(We need to keep the case to get compile-time verification of handling all of
them.)

   Unsubscribe(int64 service_worker_registration_id)
-             => (bool is_success,
+             => (PushErrorType error_type,
                  bool did_unsubscribe,
-                 PushErrorType error_type,
                  string? error_message);
 
   GetSubscription(int64 service_worker_registration_id)
@@ -149,13 +149,11 @@ interface PushMessaging {
                      array<uint8>? p256dh,
                      array<uint8>? auth);
 
-  // It returns |status| if |is_success| is true and returns |error| if
-  // |is_success| is false.
+  // We use the value of |error_type| as a flag. If |error_type| == Unknown, it
+  // means no error and returns |status|. Else, it means there is an error and
+  // returns |error_type|.
   GetPermissionStatus(int64 service_worker_registration_id,
                       bool user_visible)
-                     => (bool is_success,
-                         // |status| is meaningful when (is_success == true)
-                         PushPermissionStatus status,
-                         // |error| is meaningful when (is_success == false)
-                         PushErrorType error);
+                     => (PushErrorType error_type,
+                         PushPermissionStatus status);
 };