DL: rendersubtree attribute

Add the rendersubtree attribute (and renderSubtree property), accepting
the values "visible", "invisible", "invisible-activatable".

Will lock/unlock accordingly when the attribute is added/removed/changed
its value. Activation will set the attribute to "visible".

- acquire(), commit(), updateAndCommit() won't set/remove/change the
attribute at all. These functions (and Element.displayLock) will be
removed in another CL.
- Containment is not implied by this attribute and no DevTools error
for containment-related failures yet. Those will be
added in another CL.
- content-sizing is not currently supported. content-size CSS property
will be implemented in another CL.

See discussion:

Bug: 991095
Change-Id: I81ef6e6ff8bb5718dfb8ee09de701a08ed92062f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735579
Reviewed-by: Chris Harrelson OOO <chrishtr@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685940}
53 files changed