Fix a potential crash that can happen if a weak ptr was deallocated.

OverlayUIDidFinishPresentation is called from a weakptr, which can be nil
and it's not checked. If the weakptr is nil the call is crash as it's
called from a C++ object.

Change-Id: Idf9a6a73aded75d411e92e35a97a07a2db04b8df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795890Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695708}

ios/chrome/browser/ui/overlays/web_content_area/java_script_dialogs/java_script_dialog_overlay_coordinator.mm

@@ -75,8 +75,11 @@
       presentViewController:self.alertViewController
                    animated:animated
                  completion:^{
-                   weakSelf.delegate->OverlayUIDidFinishPresentation(
-                       weakSelf.request);
+                   __typeof__(self) strongSelf = weakSelf;
+                   if (!strongSelf)
+                     return;
+                   strongSelf.delegate->OverlayUIDidFinishPresentation(
+                       strongSelf.request);
                  }];
   self.started = YES;
 }