Add secure_context_mode_ to CSSParserContext.

This is in order to allow the CSS Parser to treat the paint() function as
invalid on insecure contexts.

This doesn't add that functionality, just the flag to the context.
I added a new enum for this to ensure type-safety, as this bool ends up next to
a few other bools like "important".

This shouldn't have any change in behaviour.

Bug: 578252
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I42c05c9a204e84a021a331f74032a73499645a51
Reviewed-on: https://chromium-review.googlesource.com/759112
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: meade_UTC10 <meade@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518016}
145 files changed