[CodeHealth] Use base::HeapArray in video_capture_overlay.cc

Replace std::unique_ptr<T[]> pattern with a base::HeapArray.

Fixed: 326458377
Change-Id: Ia212fe19592176d97f562d39bac0b293f5f6beca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5620650


Reviewed-by: Ilya Nikolaevskiy <ilnik@chromium.org>
Commit-Queue: Maria Kazinova <kazinova@google.com>
Cr-Commit-Position: refs/heads/main@{#1313331}