[Background Fetch] Improve event type exposed to DevTools.

Show the full event name to avoid confusion with extendable event result
types.

Bug: 940451
Change-Id: I6f14113d4cf7d67043df138f76ec0853562702e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619820
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Auto-Submit: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661427}
2 files changed