Mark heap snapshot test as slow

The test never fails but is just slow and runs into timeouts.

Bug: chromium:939037
Change-Id: I5b75dc4e3df15a8d0c291936497b1bce0fc034f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1528245
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641597}
2 files changed