Enable the feature BlockingDownloadsInAdFrameWithoutUserActivation

blink LGTMs:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/tXKnmZD3P9o/x5YFru_YBQAJ

Bug: 929911
Change-Id: I0d5ebe43b8c4bca13393035dd3b356ab602ea5b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625781
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662642}
2 files changed