Support |allow-downloads-without-user-activation| flag

It should be okay to support the flag without enabling the intervention feature
(i.e. when a website adapts by adding the "allow-downloads-" token, we should no
longer give deprecation warnings & should not give console errors "...invalid
sandbox flag". Both would happen if we don't support the token.).

Supporting the token will also change the result of
IsSandboxed(kSandboxDownloads): previously it was always True; now it will be
False if the "allow-downloads-" token is set. It will make the UseCounter
DownloadInSandbox keep decreasing as more and more websites are adapting to the
change. This also looks positive as it gives an idea of ongoing compatibility
risk rather than the maximum risk.

Bug: 539938
Change-Id: I002f1a556205344953dabf094849af7e1222dd9f
Reviewed-on: https://chromium-review.googlesource.com/c/1471002
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632417}
4 files changed