Remove the friend class declaration which is no longer used

The class |V8CustomEvent| does not use the API |GetFromMainWorld| now, so there is no need to declare that the class is a friend class.

Bug: 715418
Change-Id: I4a31a2ed8de4a044f19e22ccfc512c95e200d443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904560Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Marina Sakai <marinasakai@google.com>
Cr-Commit-Position: refs/heads/master@{#713737}

third_party/blink/renderer/platform/bindings/v8_private_property.h

@@ -76,7 +76,6 @@ class PLATFORM_EXPORT V8PrivateProperty {
     friend class V8PrivateProperty;
     // The following classes are exceptionally allowed to call to
     // getFromMainWorld.
-    friend class V8CustomEvent;
     friend class V8ExtendableMessageEvent;
 
     Symbol(v8::Isolate* isolate, v8::Local<v8::Private> private_symbol)