GitLab

Downloads initiated via clicking on an anchor with a 'download'
attribute should behave the same way with regard to cookie handling as a
download initiated via a top level navigation.

This CL adds logic to set the initiator origin for URLRequests initiated
in response to such downloads so that the handling of SameSite cookies
follows suit with top level navigations.

R=mkwst@chromium.org, clamy@chromium.org
BUG=648043
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2561903002
Cr-Commit-Position: refs/heads/master@{#439137}