Remove "Extensible" annotations from structs in notifications.mojom

"Extensible" annotation has no effect for structs [1] and this CL
removes them in notifications.mojom.

[1] https://chromium.googlesource.com/chromium/src/+/master/mojo/public/tools/bindings/README.md#attributes

Bug: none
Test: none

Change-Id: Ie296e48b91c28d750bf5a1f6d18f1a696c8bc0a2
Reviewed-on: https://chromium-review.googlesource.com/c/1296569Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602998}

components/arc/common/notifications.mojom

@@ -78,7 +78,7 @@ enum ArcNotificationShownContents {
 };
 
 // Flag for various feature of ARC notifications.
-[Extensible, MinVersion=17]
+[MinVersion=17]
 struct ArcNotificationFlags {
   // Bits for features.
   const uint32 SUPPORT_SNOOZE = 1;  // 1 << 0
@@ -160,7 +160,7 @@ struct ArcNotificationData {
   ArcBitmap? snapshot_image_public;
 };
 
-[Extensible, MinVersion=18]
+[MinVersion=18]
 struct ArcDoNotDisturbStatus {
   bool enabled;
 };
@@ -179,7 +179,7 @@ struct ArcNotificationUserActionData {
   bool to_be_focused_after_unlock;
 };
 
-[Extensible, MinVersion=21]
+[MinVersion=21]
 struct ArcLockScreenNotificationSetting {
   // Flag whether to show the notifications on the lock screen.
   bool show_notification;