[DevTools] Add clarification in blackbox setting tab

Bug: 818855
Change-Id: I9f626a01d9188ae25e82bc4d2a6201f20c90e621
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1622714
Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Haihong Li (Harley) <hhli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662880}
2 files changed