This patch adjusts the `SecureContext` IDL attribute to take an argument,
as we need to restrict the relevant bits and pieces to secure contexts
iff a specific flag is set. We'll unfortunately need to keep that in place
until and unless we decide that we can reasonably remove an enterprise
opt-out.

Intent to Deprecate/Remove: https://groups.google.com/a/chromium.org/d/msg/blink-dev/ANnafFBhReY/1Xdr53KxBAAJ
Spec bug: https://github.com/whatwg/html/issues/3440

Bug: 588931
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I5bedd2ca6f420a88ddbcff65e4223fad224ac0a7
Reviewed-on: https://chromium-review.googlesource.com/982625
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548391}