push_messaging: Respect ArrayBufferView's viewport at applicationServerKey

ArrayBufferView is a partial view to the underlying ArrayBuffer.
When parsing PushSubscriptionOptionsInit.applicationServerKey
given an ArrayBufferView, the partial view of the ArrayBufferView
should be processed instead of the entire underlying buffer.

Change-Id: I87156f41e50e7615bc19ac00df92a8cca2843c75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2641829
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845977}

third_party/blink/renderer/modules/push_messaging/push_subscription_options.cc

@@ -34,10 +34,8 @@ Vector<uint8_t> BufferSourceToVector(
     length = application_server_key.GetAsArrayBuffer()->ByteLength();
   } else if (application_server_key.IsArrayBufferView()) {
     input = static_cast<char*>(
-        application_server_key.GetAsArrayBufferView()->buffer()->Data());
-    length = application_server_key.GetAsArrayBufferView()
-                 ->buffer()
-                 ->ByteLength();
+        application_server_key.GetAsArrayBufferView()->BaseAddress());
+    length = application_server_key.GetAsArrayBufferView()->byteLength();
   } else if (application_server_key.IsString()) {
     if (!Base64UnpaddedURLDecode(application_server_key.GetAsString(),
                                  decoded_application_server_key)) {