[DevTools] Add buttons for HAR import and export

Screenshots:
https://bugs.chromium.org/p/chromium/issues/detail?id=904585#c2

Bug: 904585
Change-Id: I8e0eea42cb94d69c92c6a013bbdc2807b6e94a14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1333108
Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661557}
2 files changed