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This change adds the ability to export the activity log history as JSON
through an option in the 3 dots dropdown menu. The exported data is a
list of extension activities as returned by GetExtensionActivities from
the chrome.activityLogPrivate API (therefore the data might be different
than what's seen in the UI itself) but sorted by timestamp in ascending
order.

Screenshot: https://imgur.com/a/YNpyFCd

Bug: 832354
Change-Id: If891931bfa042e3f1eadecc6f6384198c4a2c6b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602074
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660693}