Add parsing support for display: contents, and minimum behavior similar to display: none

The following commit adds parsing for the display: contents value, gated after a
runtime enabled feature, and adds the basic code for display: contents elements
to not generate a LayoutObject.

BUG=657748
TEST=LayoutTests/css-display-3/display-contents-set-get.html

Review-Url: https://codereview.chromium.org/2443693003
Cr-Commit-Position: refs/heads/master@{#427437}
12 files changed