Fix WidgetTest.DestroyedWithCaptureViaEventMonitor flaky crash

ClosingView is destroyed during its call to Widget::CloseNow().
ClosingView::widget_ should not be accesses after calling CloseNow().

Bug: 895882
Test: No flaky crashes
Change-Id: I40bb3306e969332191fc7118a905eee5900f0f2e
Reviewed-on: https://chromium-review.googlesource.com/c/1283690Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600055}

ui/views/widget/widget_unittest.cc

@@ -1915,10 +1915,12 @@ class ClosingView : public View {
 
   // View:
   void OnEvent(ui::Event* event) override {
-    // Guard against attempting to close the widget twice.
-    if (widget_)
-      widget_->CloseNow();
-    widget_ = nullptr;
+    // Guard against closing twice and writing to freed memory.
+    if (widget_ && event->type() == ui::ET_MOUSE_PRESSED) {
+      Widget* widget = widget_;
+      widget_ = nullptr;
+      widget->CloseNow();
+    }
   }
 
  private: