Rename :ancestor to :host-context.

The :host-context() functional pseudo-class tests whether there is an ancestor, outside the shadow tree,
which matches a particular selector. Its syntax is:

   :host-context( <compound-selector> )

Spec link: http://drafts.csswg.org/css-scoping/#host-selector

BUG=354287
TEST=no new tests because just renaming.

Review URL: https://codereview.chromium.org/210813003

git-svn-id: svn://svn.chromium.org/blink/trunk@170016 bbb929c8-8fbe-4397-9dbb-9b2b20218538