Add a `Save` button for Background Services in DevTools.

The button is disabled if there are no events, and saves all events to a
file when clicked (JSON format).

Screenshots: https://bugs.chromium.org/p/chromium/issues/detail?id=942174#c13
Bug: 942174
Change-Id: I2c2d3600814f19050083275f52acbe40e23b0e74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1552854
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648646}