Ensure Page exists in EventHandlerRegistry::NotifyHandlersChanged

According to crbug.com/1090687 NotifyHandlersChanged() is getting called when GetPage() returns null (i.e. after Frame is detached)

To avoid the crashes this adds a check for GetPage().

Bug: 1090687
Change-Id: I154a121c96a0ac39db6a5066c8bb3940eb006838
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2234325Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776060}

third_party/blink/renderer/core/frame/event_handler_registry.cc

@@ -248,6 +248,11 @@ void EventHandlerRegistry::NotifyHandlersChanged(
     bool has_active_handlers) {
   LocalFrame* frame = GetLocalFrameForTarget(target);
 
+  // TODO(keishi): Added for crbug.com/1090687. Change to CHECK once bug is
+  // fixed.
+  if (!GetPage())
+    return;
+
   switch (handler_class) {
     case kScrollEvent:
       GetPage()->GetChromeClient().SetHasScrollEventHandlers(