Add a UA style, "display: contents", for <slot>
The spec is: https://html.spec.whatwg.org/#flow-content-3
This rule is guarded by the same runtime flag for "display: contents".
Unless the flag is enabled, the rule can not be effective because the parser does
not consider it a valid rule.
This change only affects slots in non-shadow trees since Blink excludes slots in
shadow trees from a flat tree.
Since "display: contents" is not well supported yet, the children of a slot in
non-shadow trees would not be displayed.
BUG=657748,660265
Review-Url: https://codereview.chromium.org/2466243002
Cr-Commit-Position: refs/heads/master@{#429198}
4 files changed
