M101: Prevent multiple SelectFileDialog in SessionLogHandler

Similar to crrev.com/c/3546821

(cherry picked from commit a7a8ff2)

Bug: 1305068
Change-Id: I5d7cb86d868e31683db94249a3896448178bd334
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3561204
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#987684}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3581507
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/branch-heads/4951@{#1054}
Cr-Branched-From: 27de622-refs/heads/main@{#982481}

ash/webui/diagnostics_ui/backend/session_log_handler.cc

@@ -192,6 +192,9 @@ void SessionLogHandler::HandleSaveSessionLogRequest(
       web_contents ? web_contents->GetTopLevelNativeWindow()
                    : gfx::kNullNativeWindow;
 
+  // Early return if the select file dialog is already active.
+  if (select_file_dialog_)
+    return;
   select_file_dialog_ = ui::SelectFileDialog::Create(
       this, select_file_policy_creator_.Run(web_contents));
   select_file_dialog_->SelectFile(