Only block download in sandbox without user activation

Follow the proposal discussed in https://github.com/whatwg/html/issues/3236.

Bug: 539938
Change-Id: I1841eb1f3c1f121586eed5a08e7e0e9f94ec4430
Reviewed-on: https://chromium-review.googlesource.com/c/1413377
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625878}
9 files changed